|Handbook for Agrohydrology (NRI)|
|Chapter 6: Catchment characteristics|
Catchment characteristics interact with variable patterns of rainfall and determine the character and size of runoff volumes and peak flows. This is true for both natural catchments where human activity is absent or unimportant and runoff plots upon which tillage or other agricultural treatments are being tested. Generally, there is a hierarchy of influence imposed by different characteristics, but this hierarchy is often difficult to sort out and understand. For example, where a catchment has high slopes and lime vegetation, slope will play a major role in determining the runoff regime. This regime would tend to exhibit high runoff proportions; rapidly increasing flows to high peaks and equally rapid falls. Were this catchment to be of a more linear form and were its vegetation cover to increase, then peak flows would be smaller but more prolonged and total runoff volumes would probably be less. Were the slope lower, runoff would probably be less.
In the case of catchments with low slopes, the effects of vegetation cover and microtopographic features often exert a stronger influence over runoff than the overall land slope. Local slopes are often relatively high and they may direct runoff either into basins where it can infiltrate or to channels by which it can easily leave the catchment. Heavy textured soils tend to give a higher proportion of runoff. Soil textures are related to slope as well as to parent material, and the climatic regime under which the soil formed will often have been a determining factor of the soil textural type. Where human interventions have been imposed the natural conditions of a catchment may have been altered radically; grazing, tree-felling and clearance are obvious examples. Agricultural techniques; ploughing, bunds and microcatchments are introduced to reduce runoff and usually they do, but the removal of natural vegetation and badly managed systems can have the opposite effect. Below, the main catchment characteristics and their influence on runoff are discussed.
Natural vegetation can be very important in determining runoff amounts; in many instances it is the most important influence of all, after rainfall. Areas bare of vegetation can lose more than 40% of seasonal rainfall through runoff and for intense, individual storms the loss can be much greater. Areas with dense grass cover and tree canopy cover can retain as much as 99% of the rainfall that reaches the ground. Vegetation reduces the energy of raindrops making them less erosive and intercepts rainfall which is then re-evaporated. Thus natural vegetation works against the occurrence of runoff in several ways. The same can be said of crops, but most crops provide only temporary cover and their densities, especially at ground level, rarely attain that of natural vegetation. Examples of increased runoff, soil erosion and subsequent land degradation due to the removal of natural vegetation, are common throughout the world and the literature. Consider the data presented below in Table 6.1, which compares runoff from different rangeland catchments of the same size catchments, but with various densities of vegetation cover.
Table 6.1: Comparison of End of Season Vegetation Cover and Seasonal Runoff
Note that no account is taken of other factors that influence runoff production and that the coefficient of correlation between runoff amount and vegetation cover is 0.91.
6.1.1 Measuring Vegetation Cover
Plant biomass represents the total quantity of vegetation over a given area at any time and may be variable both within and between seasons. It might be expected that the quantification of biomass is the best indicator of vegetational influences on runoff. However it is probably not the most practical index for runoff studies, because the quantification of biomass is extremely time-consuming; a large number of samples must be taken and mapped in detail and size/mass relations must be determined by the destructive sampling of trees. Generally, the assessment of total plant biomass is unlikely to be relevant to agrohydrological and water harvesting projects.
The form of vegetation; leave-shape, density, branching pattern, etc., is highly variable between species and groups of plants. Although these differences are implicit within the classification of plant species, their effect on rainfall/runoff relations are very difficult to quantify. Research into commonly-occurring trees (and crops) has been undertaken, but the results of this work is understandably limited in its applications. Moreover, biomass and vegetation cover are usually very closely correlated and the use of vegetation cover as a proxy for biomass in runoff analysis, is a legitimate substitution.
Figure 6.1 show example correlations between biomass and vegetation cover and vegetation cover and runoff.
Figure 6.1: Biomass and Vegetation Cover
Figure 6.1: Vegetation cover and Runoff
In contrast to biomass measurement, there are rapid methods of quantifying the areal extent of total vegetation cover and even though effects due to vegetation type are not always accounted for, this index makes a good indicator of the influence that vegetation can have on runoff.
Vegetation cover assessments may be undertaken on a frequent basis to study its effect on runoff, almost storm by storm. Alternatively, assessments may made only a few times each season, to understand its role in the production of runoff over longer periods. The latter case is most common, because the variation in influence of vegetation cover is not dramatic in the short term, except where wholesale removal is involved. Vegetation cover is not closely correlated to other factors that influence runoff (except perhaps seasonal changes in rainfall and temperature), but may considerably alter the soil moisture status by evapotranspiration. The relative independence of vegetation from other variables makes it a suitable factor for use in regression analysis. On the other hand because it does vary with time, unlike factors such as slope, soil type and catchment size, vegetation cover can provide an extensive range of data points for individual catchments. It lends itself well to and is often used in, runoff modelling. In general it has been recognised that the amount of vegetation cover present is a more influential factor than the type of cover. The pattern of spatial distribution of vegetation cover may also be very important.
For agrohydrological purposes, there are two conventional methods of cover measurement, though the collection of aerial photography and satellite remote sensing data are also discussed below.
A quadrat is a defined area. For field purposes, quadrats are usually permanent sampling areas retained throughout the season, within which the extent of vegetation cover is assessed. Prior to the field visit that will install them, a suitable number of quadrats is decided upon and these are placed on a site map using a fixed grid pattern. The quadrats are laid out in this predetermined, regular manner to overcome subjective bias and attain a random sampling of the area. No strict percentage sampling is required, though the more quadrats, generally the better. Ten 2m × 2m quadrats to sample 1 ha (10,000 m²) would be adequate. The quadrats are then subdivided into four sectors to facilitate accurate assessment. The quadrats can be made easily by using steel rods driven into the ground with perimeters defined by nylon rope or string. Each quarter of the quadrat is individually assessed by eye for percent total ground cover of live and dead vegetation. The overall estimate of cover is made as an average of all sectors and all quadrats. In cases of natural vegetation, any trees are included in the assessment. Assessments should be made as frequently as is feasible throughout the season, though the rate of growth will largely determine the need for inspection The estimates are to some extent subjective and it is a good idea to compare those made by different field staff, under the same conditions of cover.
This method is suitable for small areas and can be completed quickly, but projects that need to quantify cover accurately on large plots and small catchments can utilise a rapid method that is detailed below.
b. Wheel Point Method
This method is based on the simple equipment shown in Figure 6.2.
A bicycle handlebar is fitted with an extended fork assembly. The extension is made long enough to allow the passage between them of strong, sturdy spokes. The spokes, made of 5 - 10 mm diameter mild steel, are welded to a supporting plate which in turn is fixed to the axle. The forks can be any convenient length, but it is advantageous that when the observed, marked fork completes a revolution, it travels an easily recorded horizontal distance, for example 1.0 or 1.5 m. Versions with longer or shorter spokes can be made, according to whether the areas to be covered are small or large, to maintain a sufficient number of data points per unit area of catchment.
As an example, consider a plot 100 × 40 m in extent. A tape measure is stretched across one of the longer sides of the plot, 5m from the end. The apparatus is held with the marked spoke at the start of the tape and then walked along using the tape as a direction indicator.
If the marked spoke hits a bare area on touching the ground, this is called out. If it touches a vegetated area on the ground this (and if required the type and species of plant) is called out. A second person notes the call. The tape is then moved on 10 m and the process is repeated, until the whole plot has been covered, the last transect being 5 m from the other end of the plot.
This procedure gives approximately 600-700 data points for each hectare that is surveyed and takes about one hour. Less frequent sampling by using more-widely spaced transects is permissible in areas where the vegetation cover is relatively uniform. This method is also easily adapted for larger catchments and can be used in difficult and wooded terrain, with practice.
Figure 6.2: Wheel Point Apparatus
c. Aerial Photography
Aerial photographs and, more recently, satellite imagery can play an important role in the assessment of many aspects of agrohydrology, vegetation cover being one of them. Clearly, this method is inappropriate for small runoff plots, but for natural catchments it can be very useful. Large areas can be viewed quickly and catchments that are otherwise difficult to survey on the ground (those with dense tree cover, or that are inaccessible) can often be mapped much more effectively and cheaply. Additional information on surface flow routes, areas of flooding, land use, microtopography and agricultural features can also be obtained at the same time. The simplest methods of obtaining and using aerial photographs are discussed here.
Aerial photographs are used to compile maps and are often available from local survey departments. The main advantage of this is that once obtained, no further effort is needed before assessment can begin. There are, however, some serious drawbacks:
- Aerial photographs are often restricted material in many parts of the world and you may be refused them.
- When available, they are often at a scale of 1:50,000 or smaller. This is often unsuitable for detailed mapping.
- Enlargements can be made, at conventional scales, for example 1 :10,000 or 1:5,000. These are much more useful, but facilities for enlargement may not be available.
- They are almost always in black and white panchromatic format, which is poorly suited to vegetation studies. - In areas with marked seasonal differences, they will almost certainly be taken during the dry season when conditions for photography are best, but little information is available on vegetation or crop cover.
- Photographs for mapping purposes are not taken frequently and different sets of photos may be decades apart, ground conditions may have changed radically since they were obtained.
If suitable orthodox photographs can be obtained, fine, but it is well worth considering obtaining your own. This is much simpler and cheaper than may be expected and has several advantages:
- Photographs can be obtained at the most useful scales. The use of slides allows a range of scales to be obtained.
- Colour or infra-red photographs can be obtained (though the latter film may be difficult to buy and have processed).
- If slides rather than prints are taken, these are very useful for projection, mapping and conversion to prints.
- They can be taken at critical times during the season.
- Particular sites or areas can be selected.
It is unlikely that the precision of scale and lack of distortion of map survey photographs can be equalled, but in most cases these are of minor importance compared to the advantages listed above. The general conditions to obtain good quality photographs economically are as follows.
Any light aeroplane (2-3 seat) can be used. Enquire if a glass panel can be easily inserted into the floor to give a vertical view or if this modification has been made previously. If not, a door will have to be removed and a wind shield fitted (this is not unusual, but vertical photographs will be more difficult to take). Plan the most economical route to all sites and submit a flight plan to be discussed with the pilot. As a guide, three sites situated within a 50 km radius of the airport can be covered in little more than one hour.
A good 35 mm single lens reflex camera (through-the-lens viewing is essential) is adequate. The type of lens is a point of preference and the aims of the photography will play an important part in the choice, because although the focal length of the lens will determine photographic scale and is technically important, the ease of use in the confined space of the cabin, the ability to work rapidly and the need for different scales may be paramount.
A 70 - 210 mm focal length zoom lens will probably be suitable for most occasions since it gives approximately × 1.4 to × 4.0 magnification. This flexibility of magnification means there is no need to change the lens to cover different sized areas efficiently. Unless very small areas are to be studied, a 35 - 150 mm zoom would also be suitable and in this case slightly wide angle views can also be obtained. Another advantage that zoom lenses have is that their magnifications obviate the need for the aircraft to change altitude.
Figure 6.3: Scale of Vertical Photograph Over Flat Terrain
In a small plane this can take a long time and can add considerably to the cost when several sites are being photographed at different scales. It is important to remember, however, that with zoom lenses, the exact focal length currently in use may be unknown and the scale of the photograph cannot be calculated, unless ground reference points of known dimensions are available. Sometimes it is best to preview the area to be photographed and tape the focal length of the lens in a fixed position with adhesive tape. It is not necessary thereafter to be continually manipulating the lens and the tape prevents it from accidentally sliding out of position when held vertically downwards.
Lenses of single focal lengths overcome these problems, but time must be allowed to change them and only a limited range can be used. As sunny conditions will undoubtedly prevail, film and shutter speeds are not usually a problem. A large depth of field is not needed, so wide aperture stops can be used to give high shutter speeds. To prevent blurring due to vibration, 1/500 th or 1/1000 th of a second exposures are recommended. Fast films (ASA 400 and above) should not be necessary and may not be available nor be easily developed. They tend to be grainy when enlarged.
Films should be at hand and clearly marked with date and location. Ground location markers may be necessary for site identification. At 2,000 - 3,000 feet (650 - 1000 m) above ground level, a good operating altitude for light aircraft, strips of white paper about 30 cm wide and 10 - 20 meters long are clearly visible. If they are set to known lengths, they make good ground reference markers for obtaining scales.
Photographs taken over terrain of widely varying altitudes exhibit varying scales and tilted photographs have nonuniform scales.
Table 6.2 below gives a guide to ground coverage with various altitudes and focal lengths . This is the actual area on the ground that will be captured by a 35 mm negative or slide diapositive of size 25 mm × 36 mm.
Table 6.2: Ground Cover Area for Different Altitudes and Focal Lengths
The largest area covered in the table above is between five and six square kilometres. This is the size of a small catchment, but details on the ground are not easy to see.
A mosaic of photographs, or a continuous transect of frames that cover a large area but which also show fine detail are possible, but not easy to obtain. Transects can be planned on maps and air speeds calculated so that photographs may be taken at counted time intervals, without taking account of the view below. In practice, pilots find it difficult to keep a straight course with only a visual marker on the horizon and airspeeds vary due to wind. Drifting causes further problems. To some extent trial and error must play a part, but care and acute observation must be exercised to obtain reasonable coverage using transect flight paths.
d. Satellite Remote Sensing
During the last two decades or so, satellite imagery has become more widely used for water resource projects, among others. The importance of such imagery cannot be overstated, but the area of satellite image analysis is a very complex one and can only be covered here, very briefly.
The three main factors that dictate the usefulness of satellite imagery to a project are:
These define the potential repeat period for the coverage of an area. For example the polar orbiting NOAA satellites can obtain imagery at least once per day per satellite. The Landsat satellites have a repeat period of about two and a half weeks. The altitudes of various satellites are also greatly different and will affect ground resolution and size of coverage.
Satellites, their orbits and sensors are designed for particular purposes. For example, Landsat satellites were designed for terrestrial research, Seasat for oceanographic study and Metsat for meteorological investigation. Different sensors are used to give the best results within a particular environment and may have restricted use outside that environment. Visible, infra-red, near infra-red and micro-wave (radar) sensors are commonly used, each of which is most suited to a particular application.
The size of an object that can be detected from a satellite, depends upon the resolution of the sensor, this may vary from a few metres, or even less, to several kilometres. It will also depend on the kind of sensor that is deployed and the spectral characteristics, shape and surroundings of the object that is viewed. In general, the area of coverage is smallest when resolution is finest, but in all cases coverage is "regional".
Imagery comes in two formats; hard (usually photographic) copy and computer compatible tapes (CCTs). The former may be colour (a combination of bands) or black and white (single band) and is relatively cheap and easy to work with. It will be purchased in a form that has been geometrically corrected for changes in satellite velocity, altitude, attitude and for Earth rotation and curvature. CCTs must be viewed using special computer facilities, desk-top versions of which are now widely available. These images can be extensively processed and enhanced and are the source of hard copy images. They and the equipment to process them are usually very expensive, though research institutions can in some cases, gain the image material for no, or little, cost.
Vegetation cover assessment is commonly undertaken using satellite imagery and the physical characteristics of catchments, their soil moisture status and hydrology can also be studied. However, the selection of satellite, imagery and waveband; the selection and utilisation of techniques for analysis is extremely complex and specialist literature should be consulted.
Interception can only be loosely defined as a catchment characteristic as it is the combined effect of several influential factors such as rainfall, climate and vegetation cover. However, in other respects it falls conveniently into this chapter and so is discussed here.
Losses from interception, the rainfall that collects on vegetation and is re-evaporated, can be highly variable and depends mostly on vegetation type (size, shape and disposition of leaves and branches); rainfall amount, intensity and drop size; wind speed, temperature and eddying. Interception is difficult to measure, especially for crops. It can be attempted by placing rain gauges under vegetation either randomly to sample average interception, or by the selection of specific target areas. In wooded catchments, rain gauges should be attached to tree trunks to assess stem flow, as in Figure 6.6 below, but with multi-stemmed vegetation this is very difficult.
Figure 6.6: Stemflow Measurement on trees
In Figure 6.6, in addition to free-standing gauges under the canopy, a peripheral collector is wrapped around the trunk to direct flow into a single rain gauge that is covered.
Empirical work has led to estimates of losses of 10 - 20% of seasonal rainfall and deduced storage capacities of 0.8 to 1.5 mm of rain per storm. Equation (6.1) describes an empirical interception relation and Table 6.3 gives examples for various crops for a 25 mm rainfall.
I = (Si + Etr) (1 - e-kP) where (6.1)
I = total interception
Si = storage capacity per unit of the area
E = evaporation rate
tr = duration of rainfall
P = amount of rain k = 1/ (Si + Etr)
e = base of natural logs.
In terms of runoff studies, the situation regarding interception is even more complex. It is usually lumped with rainfall storage due to ponding and infiltration for runoff modelling purposes, where it is assigned a purely notional value.
Table 6.3 Interception Losses from a 25 mm Rainfall
6.3.1 Catchment Size and Land Slope
Catchment size is an important influence on absolute values of runoff amount and peak flows and is an essential parameter in runoff formulae that predict these hydrological characteristics. The determination of catchment size will be straightforward in most cases. Runoff plots are usually bounded by bunds or galvanised metal sheets that prevent runon from outside the proscribed catchment area. Natural catchments will usually be defined by clear patterns of drainage and topographies that show the limits of a catchment area. In some cases these details will be available from topographic maps, in others aerial photography may be the most suitable source of information. In general, the size of a catchment that is monitored will be limited by the practicalities of the natural or artificial controls that can be used as flow measuring sections, the aims of the project and the resources that can be invested in obtaining runoff data. Catchment size is not a good indicator of percent runoff; influences such as land use, soil type and slope are more important, but in terms of absolute values catchment size is very important. It is unfortunate that a simple proportional reduction or increase of runoff cannot be deduced from the size of a catchment, even where catchment conditions are ostensibly the same ( see the section on slope and microtopography below). To illustrate the difficulties in making assumptions on runoff proportion and catchment size, Table 6.4 gives percent runoff for large catchments, R2 0.12 and is not significant.
Table 6.4: Relation Between Catchment Area and Runoff
Figure 6.7: Catchment Size versus Runoff from Experimental Plots
The scale of these catchments is larger than is often studied for agrohydrological research, but Figure 6.7 shows a graph of catchment size versus percent runoff, the data for which were obtained from experimental plots and catchments sited in and around farmers' fields. These plots are divided into three groups with similar catchment conditions, to remove any influence that different conditions could exert on runoff. The conditions are crop (squares); rangeland (triangles) and fallow (circles). The R2 of the analyses were 0.108, 0.066 and 0.602 respectively and none of the relations were significant.
Suitable Catchment Sizes for Runoff Plots
a. Plots Representing Farmers' Field Conditions
In many cases, it is important to collect data on the actual losses of rainfall, as runoff, from farmers' fields. These data show whether such runoff is important and if so, provide the information to design preventative measures. Observations of runoff which do not involve actual measurement are notoriously misleading and anecdotal evidence to estimate runoff amounts should not be used. Runoff channels and other evidence do not provide accurate information on volumes and frequencies and no decisions should be made on the basis of their observation
It is important at the outset of runoff plot experimentation, to define the most appropriate size of plot. This size will depend on several factors, but the most important is that it should be representative of actual field conditions. The use of very small plots has several advantages; many replicates can be built, they are easy and cheap to instrument, and they occupy only a small portion of any research area. It is unlikely, however, that a plot that is only 20 square metres in extent, for example, can be used to represent the runoff regime of a farmer's field. The actual dimensions and shape of the any runoff plot are best determined by the aims of the research agenda, the finance and equipment that are available, the remoteness of the site etc., but it is essential that the following considerations be made:
- The plot should include representative field topography, so that within the plot, the overall land slope of the field should be included. Slopes influence the velocity of runoff and will affect opportunities for it to infiltrate and overwhelm ploughed ridges. Because runoff velocity increases by the square root of slope, small differences in slope between plots will not lead to large differences in runoff velocity or amount. Low overall land slopes greatly increase the storage capacity of ploughed ridges and bunds (see chapter 7 on water harvesting for details), thereby reducing the possibility of runoff.
- Within the plot, the microtopography (the small-scale ups and downs and ploughed ridges and furrows) of the field should be included. This is especially important in flat areas where microtopographical features may have local slopes greatly in excess of the overall land slope and may be very important in inducing runoff. The redistribution of this local runoff (which may constitute net runoff from the field) will be determined by the size, pattern and distribution of microtopography. This can exist as basins and mounds or ridges and channels, the former could be expected to impede runoff, the latter to assist its passage to the field margins.
- Ploughed ridges and furrows will inevitably leave the contour at some point and encourage water movement to low-lying areas. This should be taken into account when plots are being planned and runoff should not be impeded by the artificial boundaries of the plot.
- Another important reason to include representative rnicrotopography is its potential to indicate changes in soil texture and nutrient status. Differences in infiltration rates, water holding capacity, soil depth and soil chemical characteristics may be present, resulting in a local variation of runoff production and crop performance. The inclusion of microtopography within runoff plots will not only influence the physical processes of runoff, but will also allow agronomic sampling procedures to assess more accurately, the effect that these have on crops.
- It is important to note that although in land-levelled fields natural microtopography may not be evident, residual soil variability will still be present and may have an important influence on crop growth. Plots that are used to measure runoff from farmers' fields should cover at least 10% of the total area, more where fields are less than 5 ha in extent. A 30 cm H flume will have an adequate capacity to cope with flows from plots of around 0.5 to 1 hectare. Plot length should exceed 80 m where field-scale runoff is to be defined and plots should be representative of field slope and topographic conditions. They should be ploughed and planted according to the farmer's usual methods. Where similar plots are used to measure runoff from naturally vegetated areas, a representative cover should be included. Very bare plots of 0.5 hectare may be expected to give flows close to the capacity of a 30 cm H flume and a larger instrument may be preferred.
b. Within Field (Small-Scale) Runoff Plots
Plots built to estimate runoff on small-scale water harvesting and tillage schemes are much simpler than those built to represent farmers' field conditions. They are usually smaller in dimension than any microtopography that may be present.
In these instances, it is usually not difficult to place plots to measure runoff on any slope that is desired. Edge effects can be influential and it is important that boundaries do not channel runoff to the collection tank in an unrealistic manner. Rain falling directly into impermeable gutters, drains, etc. should be taken into account.
Runoff will exploit very small elevation differences and sheet flow is quickly converted into channel flow. If the aim of the experimentation is to promote the even redistribution of runoff to the crop rooting zone, this is an important fact to note.
Ploughed ridges and furrows play an important part in influencing runoff in these circumstances and dead furrows may be a consequence of ploughing technique. They can store a considerable amount of runoff (typically about 500 litres or 0.5 m³ per 10 m length) and their location can make a significant difference to runoff measurement, especially for small runoff events.
It should be noted that such small plots may not behave as on the research station if they are transferred and installed as extensive systems on farmers' field, where pronounced microtopography may exist. The importance of placing runoff plots in full knowledge of the effect of microtopography on runoff measurement cannot be overstated.
In the first case (location Figure 6.8) average seasonal percent runoff from the mounds was measured as 29.0 %, while the runoff from the crop plot (marked on Figure 6.8) and which measures 100 m × 40 m, was only 4.5 % on average, over three seasons. Slopes of the microtopography were about 5%, of the large plot about 0.5%.
In the second case (location Figure 6.9), local runoff due to microtopography, from the ridges to the channels, was in excess of 15% whereas average runoff from four, 100 m × 40 m plots located on farmers' fields, but not shown in Figure 6.8, ranged from 1.7% to 4.5% over three seasons. Slopes from ridges to channels ranged from about 3-8%,
large plots slopes were approximately 1%. If, in such cases, the results of runoff measurement from the small plots were extrapolated to estimate net runoff values from the whole field, they would lead to a gross over-estimation.
In practical terms this over-estimation might lead to the supposition that the prevention of runoff was of paramount importance and costly (to the farmer in terms of labour input for reward from increased yields) control measures might be implemented. Where rainfall amounts are regarded as marginal for crop production, these results might also suggest that additional supplementary water should be obtained by water harvesting. The apparent runoff efficiencies of 15 - 29% indicate a high runoff efficiency, and it might be expected that an extra 100 - 125 mm per season could be provided on the basis of a 1:1 crop to water harvesting area ratio. The actual runoff efficiencies of around 2 - 4% for the larger plots show that this is not the case and 10 mm might represent the realistic supplement that would be available for crops (ratio 1:1), unless the harvesting to crop area ratio was very large.
Figure 6.10 shows a typical simple installation for the measurement of runoff from field microtopography.
Figure 6.10: Installation to Measure Runoff from Field Microtopography
c. Natural Catchments
Natural catchments are usually larger than those that are artificially defined for the purposes of runoff measurement. They frequently include areas with different land slopes, soil textures, vegetation and microtopography. In areas with abrupt changes in geology, different densities of stream networks are often exhibited. Natural catchments are, therefore, more difficult to characterise than artificially bounded catchments. For the purposes of study they may have to be divided into subcatchments each with a more homogeneous nature. Runoff may then be measured at locations to include each of these relatively homogeneous areas.
Field orientation is particularly important with regard to water conservation measures that may be attempted on agricultural land. Fields are often defined according to convenience, exploiting useful land marks such as the position of roads and access, rivers and natural features. They are rarely oriented with runoff losses and methods of runoff prevention in mind. Figure 6.11 shows a typical semi-arid agricultural landscape.
A study of the photograph and the features of drainage shows that most fields are oriented so that one corner is at the highest topographic elevation. Few of the boundaries are parallel or at right angles to the overall land slope and the natural drainage. In practical terms, this means that when a farmer ploughs, he or she will always plough such that ridges and furrows provide channels that encourage runoff. To plough along the contour would necessitate a start in the highest or lowest corner, and ploughing for very short distances. The length of ploughing would gradually increase until the full diagonal width of the field was attained, then the distances would decrease until the farmer eventually reached the opposite corner from where he or she had started. This would be a very difficult and inefficient exercise from the viewpoint of ploughing, but cultivation would be on the contour, disregarding local variations, and would inhibit the natural flow direction of runoff. If contour bunding were to be practiced, similar difficulties would be encountered.
The boundaries set for fields, in areas of agricultural activity where the effects of urbanisation are small, are often those of roads and tracks. The directions of these roads and tracks are not often exactly along the contour. They remove natural vegetation, cross natural drainage systems, redirect runoff and concentrate it into ditches, under culverts and bridges. This leads to the disruption of the natural drainage, the concentration of flow and in many cases, serious problems of soil erosion.
The problems of field orientation are complex. They involve land ownership, the freedom of access and many other social issues, as well as a consideration of the physical environment and the behaviour of drainage. The field studies of most projects will be sited upon land that is already allocated and used for farming, so few opportunities for the implementation of new allocations will exist. However, the influence of field orientation is an important factor to note when field sites are being selected and where the opportunity exists, serious consideration should be given to the siting of new fields with a favourable aspect to natural drainage. The problems of contour cultivation and the effects of local microtopography on such practices are discussed in more detail in chapter 7, Water Harvesting.
Antecedent soil moisture conditions strongly influence the rate at which rainfall infiltrates into the soil and contribute to the processes of runoff production. Soil moisture levels at any time are the result of a combination of several factors, mainly: the time elapsed since the last rainfall; the rainfall amount and intensity; the climatic conditions that have prevailed since rainfall; the type and stage of development of vegetation and soil texture and depth. Soil moisture levels can be highly variable both between and within periods of a particular meteorological activity. A high degree of spatial variability of soil moisture conditions may also be encountered.
Soil moisture levels can be estimated by accounting procedures that balance the infiltration of rainfall against losses by drainage and evapotranspiration. The calculation of evapotranspiration (Et) by different methods is discussed in Chapter 8. A commonly used accounting procedure derives an Antecedent Precipitation Index, by the application of an estimated factor for Et losses on previous rainfall. It is generally assumed that the rate of reduction of the soil moisture reserves is logarithmic, the rate falling as the availability of water decreases. The mechanisms by which antecedent soil moisture effects runoff are highly variable from soil to soil, but the general assumption equates higher proportions of runoff with higher levels of soil moisture. This reflects the behaviour of infiltration rates under increasingly moist conditions. Figure 6.12 illustrates changes in antecedent soil moisture according to rainfall.
Figure 6.12: Change of Antecedent Soil Moisture Levels as shown by the Antecedent Precipitation index
It is important early in a study to determine the precision to which antecedent soil moisture needs to be measured or calculated. In situ measurements can be time-consuming and calculations of Et usually necessitate the collection of a wide range of meteorological information (see Chapter 8). General indicators of soil moisture status may be adequate in some instances, but for use in, for example, regression analysis against event runoff data, estimates of actual values are necessary.
Among the other influences on runoff that can be important is the geological nature of the area under study. It affects runoff in three main ways:
1. Lithology. Particular rock types that are exposed at the surface of the ground can have a profound influence on runoff, but generally large areas of exposed rock are not common. Impermeable rock surfaces such as granite, gneisses, shales etc. can produce very high percentages of surface flow and may be locally important as sources of runoff. In semi-arid areas, and where these rocks are highly fractured, they not only lead to rapid runoff from their impermeable surfaces, but can also provide ground water that prolongs stream flow beyond the normally short period of flash-flooding. Permeable lithologies such as limestone and porous sandstone can limit surface flow to brief periods, only attained when ground water levels are extremely high. Perennial springs may occur where they overlie impermeable layers. According to its permeability, the geology of an area will determine surface drainage density, stream channel length and catchment shape.
2. Soils. The most widespread effect on runoff, of geology, is through the type of soil that it engenders. Granite, sandstones and quartzites produce sandy soils with relatively few nutrients and high rates of infiltration. Shales and basic igneous rocks usually give rise to relatively impermeable clays, though the humidity of the climate will determine the processes of weathering and erosion, and the type of soil that is subsequently formed.
3. Topography. Topography is also the result of geology and climate which determine land form, slopes and local microtopography.
b. Stream Density
Stream density is an index of the concentration of a drainage network within a catchment area. It will not be an important factor when small runoff plots are studied, but is used frequently as an independent variable in regression analysis for natural catchments. It should be noted that larger runoff plots, for instance those greater than about 0.5 hectare especially where they constitute a portion of a large field or a natural catchment, may well exhibit a stream network, though in arid and semi arid regions stream flow will be ephemeral. Such networks exhibit themselves as microtopography and may be influential in determining the runoff efficiency of the catchment, as they intercept sheet flow and channel it to the catchment outlet. The measurement of discharge by sheet flow alone is more difficult to achieve, as it is prone to retention by vegetation, ponding and subsequent infiltration. In origin, stream density is closely related to structural geology, Ethology, slope and climate. In general terms the greater the density of a stream network, the greater the percentage runoff for any given rainfall, because stream channels conduct runoff efficiently they lead to high, sharp peaks and rapid recessions. Relatively complicated systems of stream hierarchy are used to derive a stream density index in hydrological analysis, but they are somewhat beyond the scope of this book. A simple but appropriate index that can be used for regression analysis is length of stream per unit area (e.g. km km-2), though the correlation of such a stream index with other catchment characteristics must be considered before use in regression.
c. Human Factors and Agriculture
Human influences on runoff can be very great and they work at many scales. The wholesale destruction of huge forested areas has led to disastrous flooding and extreme environmental degradation. A list of the most important influences that affect the amount of runoff from the land may include:
Reduction of flood plains
Draining of marshes and swamps
Arable agriculture appears less harmful, perhaps, but its effect on the hydrological nature of much of he world's surface is profound.
All costs of locally made equipment are approximate. The costs of raw materials and especially labour are highly variable from country to country, but a good idea of cost magnitude can be gained from the figures quoted below. The costs of manufactured equipment are based on 1993 prices. Shipping, agents' fees and fluctuations in exchange rate cannot be taken into account.
Typical Approximate Cost in $ US
2 - 3 Seat light aircraft
300 - 500
4 - 5 Seat with global navigation
400 - 700
(sufficient to cover 2 -3 sites within about 30 km radius)
Wheel point apparatus
30 - 60