|Handbook for Agrohydrology (NRI)|
|Chapter 6: Catchment characteristics|
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.