|Handbook for Agrohydrology (NRI)|
|Chapter 5: Soils and soil moisture data|
Infiltration is the process whereby water on the soil surface percolates downwards. Infiltration rates represent the speed at which this percolation occurs and are expressed in mm in-l. The maximum rate for any given soil condition is termed the "infiltration capacity". The controls on infiltration rates are many: soil texture, cavities and impermeable layers, vegetation cover, air spaces, soil wetness and topography all may be influential. Land use and cultivation can also be extremely important as they affect the quantity of suspended material in surface water which in turn influences rates of infiltration, because the suspended material blocks pore spaces and increases runoff. Measured rates of infiltration lump the effects of all these influences together.
Runoff is the proportion of rainfall that does not infiltrate, at least immediately, but infiltration rates vary greatly with time, especially during rainstorms when the soil becomes progressively wetter and as rainfall intensities vary. Thus the supply rate of water to the soil surface and the rate at which it infiltrates are never constant. Despite the fact that infiltration rates represent a gross generalisation of soil/water behaviour, they are often important components of hydrological and runoff models.
Extensive field trials have shown that infiltration rates decrease with time, in the general form:
I = (aTn + b) where (5.4)
I = infiltration rate
a, b and n are constants and
T= Time elapsed
Thus infiltration rates are exponential. As the rate of infiltration decreases it approaches, and sometimes achieves, a terminal value. For clay soils, the value of 'b' in equation 5.4 may be almost zero, while for sandy soils it will be much greater; soil texture plays an important part in the determination of infiltration rates. Infiltration rates are used not only to estimate the likelihood of runoff, but are also quoted as a general soil characteristic, but it should be noted that the infiltration rates of soils are notoriously spatially variable, even over distances of a few metres.
Figure 5.17: Variation of Infiltration Rates Over Small Areas
Variability is important because it can be associated with microtopography in fields and cultivation practices which alter the location of topsoils. For example Figure 5.17 shows the variation of infiltration rates of 20 tests undertaken over an area only 15 m square, on an apparently uniform soil. Variation will affect the redistribution of runoff at the local scale and will be influential in determining the soil moisture that is available to plants. Important differences between rates on cultivated and uncultivated land are likely to be seen.
Figure 5.18 shows example infiltration curves for different soil textwal types: sandy, loamy and clay soils.
Figure 5.18: Example Infiltration Curves for Sandy, Loamy and Clay Soils
Three methods of measuring infiltration are discussed below.
5.3.1. Equipment and Methods of Measurement
a. Double Ring Infiltrometers
Double ring infiltrometers consist of two concentrically placed rings, both filled with water. The rate of infiltration into the ground within the inner ring is measured, while the water in the outer ring provides a buffer to ensure the direct downward movement of water below the inner ring.
Infiltrometers can be purchased, but as they are simple to manufacture to specific requirements, this may be preferred. They can be made by using metal water pipe cut to a suitable length and given a bevelled, sharpened edge at one end. Example dimensions are: length 30 cm, diameter 60 cm, the smaller ring should be of the same length, but half the diameter, also sharpened. The two different sized rings are inserted into the ground, the depth of insertion will depend on the hardness of the soil. A more or less constant head of water is maintained at a measured and marked level (10 cm is suitable) above the ground surface. Water is poured into both rings until it reaches this level and then another 1 litre of water is added. When the water in the inner ring falls to the marked level another litre is poured in, to compensate for infiltration. If infiltration is slow, only 0.5 litre need be added; the water in the outer ring is also kept to the level and generally it will take one or two hours to reach a constant (terminal) rate of infiltration. Figure 5.19 shows the installation of the double ring infiltrometer.
Figure 5.19: Double ring Infiltrometer.
The accuracy of the data collected from infiltrometers may be affected by the insertion of the rings, which can alter the physical characteristics of the soil. Soils with macropores, burrows etc. may show extreme rates of infiltration and prove to be unsuitable for study by this method. Crusted soils can be prepared by cutting the crust with a razor blade and inserting the rings through the cuts. The gap between the ring and soil may be sealed with gypsum paste or hydraulic cement.
The major difficulty with data that are obtained by the double ring method however, is that they do not represent infiltration under rainfall/runoff conditions: rainfall impact may increase or reduce infiltration at the soil surface; rain storm intensities vary greatly and the standing heads of water used by these infiltrometers are not usually representative of real conditions. Runoff flows away and is not impounded. The spatial variability of infiltration capacities is common and results may be relevant only to very small areas. It is important to note, however, that data from double-ring infiltrometers are widely used and it may be essential to collect this information if comparisons between a range of sites and soils are to be made. The dates' main value is as a reference for comparison rather than the provision of absolute, true infiltration rates, though these may be of direct application to irrigation practice. To overcome the unrealistic results that double ring infiltrometers often provide, sprinkle infiltrometers have been developed. An approximate replication of natural rainfall can be obtained by the use of sprayers and sprinklers. Sprinklers can be complex systems that simulate duration, rate, drop size etc., but even the smallest versions of these instruments are not really portable and often require large volumes of water to operate. Knapsack sprayers are low cost alternatives that can be easily transported and when used carefully, provide relatively good data.
b. Knapsack Spray Infiltrometer
Spray infiltration tests using knapsack sprayers improve upon the results from ring infiltrometers by minimising the effect of a standing head of water and by applying the water as a spray. Further emulation of the rainfall/ infiltration process is not attempted. Although this method is only a rudimentary attempt to simulate rainfall it does provide an improved technique which is portable, easily replicated and inexpensive. Typically, a quadrat is sprayed at a designated rate which is reduced upon the evidence of standing surface water. Details of the method are as follows:
An area 1 m square is marked and within it a 50 cm × 50 cm × 5 cm deep wooden quadrat is placed centrally, pushed 2 -3 cm into the ground. The central quadrat is divided into quarters. Wind shields should be provided around the site if necessary. The sprayer is best fitted with a fan nozzle to provide a wide, even spray. The 1 m² quadrat is sprayed with an even application every 30 seconds. The period of pumping to prime the sprayer and the duration of application are regulated (for example pumping and application for 5 and 10 seconds respectively) to give a similar intensity spray each time. When water is seen standing on two or more of the central quarters, the next spray is omitted. This may be continued for an hour or more, or until a recognisable uniformity of application indicates stability of the infiltration rate. The amount of water delivered is quantified by repeating the spray test procedure into a measuring vessel before and after the test. The total volume of water that can be applied during the test will depend upon the nozzle aperture, but depths of 40 mm can easily be achieved. Variations on this method may be designed to account for local conditions.
This method, though not representing true rainfall conditions, is relatively simple and easy to replicate and the data obtained discriminate clearly between soils of different texture. The rates also resemble those that might be expected under rainfall conditions. Figure 5.20 shows a vertical view of equipment layout used in running tests with a knapsack sprayer.
Figure 5.20: Knapsack Infiltration Test Equipment (Vertical View)
Testing may be undertaken on cropped, range or fallow land. The simulation of ploughing effects can be achieved by digging the soil over; this may be necessary in regions that have distinct dry and wet seasons. In these areas testing is most suitably undertaken during the dry season, when the influence of rainfall and high levels of antecedent soil moisture are not evident. It is likely that fields will not be cultivated at this time.
c. Sprinkler infiltrometers (Rainfall Simulators)
Unlike double-ring infiltrometers, sprinklers are not limited to the study of infiltration rates. They are often used to investigate such influences on the rainfal/runoff process as soils, slopes and tillage practices and may be used to measure rates of soil erosion. It is convenient however, to discuss this type of equipment here. Sprinklers attempt to simulate the process of rainfall, while allowing a control over the amount, intensity and drop size of applications in a manner that is not possible with natural rainfall. Though they represent rainfal/runoff conditions more realistically than ring infiltrometers and portable knapsack sprayers, sprinkle infiltrometers also have limitations. An important question to ask when considering sprinkler design is which of the main rainfall characteristics should be simulated most closely: for example drop-size or terminal velocity? Variations in intensity or uniform applications? Difficulties exist even in measuring the characteristics of the natural rainfall that is being simulated.
Sprinkler design can be esoteric, and "production models" are not common. Most sprinklers cannot be considered at all portable, they are too large and even small sprinklers may need compressors, pumps or a mains water supply to operate. Large, boom type sprinklers are usually confined to agricultural research stations where there is a plentiful supply of water and where it is preferable to maintain tight control over the soil, slope, cover and tillage variables under investigation. As runoff and soil loss are related to rainfall kinetic energy per unit area, this is a useful parameter by which to make comparisons of sprinklers. In general two types of rainfall simulations are adopted.
The first type are those using nozzles which most easily reproduce a drop size distribution akin to natural rainfall, but which have complex intensity reducing systems.
The second use drop-formers and are simpler in construction, but the drops do not reach terminal velocities until falling 5 m or more.
It is far beyond the scope of this book to describe the many individual types of sprinklers that have been developed, often for particular research purposes, and which are not easily nor commercially available. A comparative list of such equipment is given in Part 1 of monograph no. 9 of the American Society of Agronomy and Soil Science Society of America (1986). Most sprinklers of the "portable" kind apply water to relatively small areas, usually about 1 m². The problem of spatial variability of soil characteristics is therefore often as great with these devices as it is with ring infiltrometers and knapsack sprayers, and the extrapolation of results beyond the locality of application needs careful consideration.
The Type F infiltrometer has been used in the USA on larger plots of approximately 2 m × 4 m in size and is not regarded as portable. the type FA operates over a smaller area. The manner of operation to obtain infiltration data is recommended as follows:
- First, several calibration runs are undertaken with the test area covered by a waterproof sheet, to measure the simulated rainfall application rate.
- A test run is then started with the sheet removed and continued until the rate of runoff becomes constant (as does the rate of infiltration).
- The analytical run is started when application ceases and runoff stops, but before any recovery of the infiltration capacity has occurred. The rate of infiltration is therefore constant throughout the run. Runoff is measured. Any difference between the application minus infiltration and runoff is due to depression storage and detention storage.
- The effects of these on the runoff process may be investigated for various land conditions, if required.
The relation between runoff and infiltration data is discussed further in chapter 8, Data Analysis
The most important components of sprinklers are the nozzles that control the characteristics of the water that is applied to simulate rainfall. The testing of suitable nozzles on an individual basis was undertaken by the Ministry of Agriculture, Harare, Zimbabwe as part of research into the SLEMSA soil erosion model (see chapter 3), over 1 m² areas. Of the various nozzles tested, several performed well and details are given in the Ministry's Research Bulletin no 25 (1980). The construction details of a mobile sprinkler taken from this bulletin are given Appendix C.