6.3 Catchment size, slope and topography

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.