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Runoff, erosion and sedimentation: prediction and measurement


Runoff forms

There are two situations when runoff will occur. If the intensity of rainfall exceeds the infiltration rate at the ground surface, ponding will lead to surface flow. Alternatively, when the soil surface is saturated there will be surface flow when the rainfall intensity exceeds the percolation through the whole soil profile, a combination of downward movement to groundwater and lateral movement to seepage flow.

The rate of runoff is required for the design of drains, canals and other channels, and for the prediction of water levels in streams and rivers. Quantity of runoff is required when storage is involved for irrigation, power generation, river transport etc. Estimates of quantity must include annual or longer term variations, minimum yield and reliability.

Quality of runoff is increasingly a matter of concern; chemical pollution, from health and environmental aspects; sedimentation, because of interference with drainage, land use, irrigation and power generation. The prediction model CREAMS (Chemical Runoff and Erosion from Agricultural Management Systems), currently widely used in the USA, includes estimates of chemical pollution and sediment from agricultural sources.

Runoff measurements

The measurement of runoff, in the field, is generally carried out using current meters and calibrated or rated channel cross sections, flumes or standardized weirs, together with water level readings, often by automatic recorders, to give a continuous height record which can be correlated to flow.


Parameters of soil loss

The old concept of measuring soil loss in terms of tonnes/ha is giving way to more realistic assessments of loss of productivity. Unfortunately the links between loss of soil and the reduction of yields are only poorly researched and documented, although several projects are now seeking to remedy this deficiency (such as the AGLS programme).

N. Hudson, SILSOE Associates, Bedford, UK

FAO Soils Bulletin 68, 'Field Measurement of Soil and Runoff', forms the source of the summary paper presented at the Workshop, and is available from FAO.

The approach to soil loss is different for agronomists and hydrologists. Agronomists are primarily concerned with on-site effects of soil loss on farming, i.e. where the soil is coming from. Hydrologists and water engineers are more interested in the off-site effects of erosion where the soil goes to, and the effect on water supplies and sediment transport. There is a quantitative difference between the two. All the soil that leaves the field is lost to productivity, and only a minute proportion may have a payback in terms of benefits from soil deposited elsewhere. But not all the soil lost from fields becomes sediment in streams and rivers, because some is held in conservation works and some by vegetation, or in pools and hollows. The proportion of gross erosion from the land which passes into the stream system is called the delivery ratio.

Measurements of soil loss

Approaches to soil loss measurement include reconnaissance methods, which are cheap and simple, and depend on the measurement of changes in soil surface levels, and volumetric methods which involve three-dimensional observations of the lengths and cross sections of rills, gulleys, etc. Another volumetric approach is to estimate the amounts deposited as outwash fans or in catchpits or reservoirs. Aerial photography is a useful tool here.

Runoff plots are expensive, and usually ineffective and, because natural rainfall varies greatly from storm to storm and year to year, this adds to the difficulty of assessing results from these plots. Artificial rainfall simulators can eliminate these variations and speed up the collection of data.

The cost of construction and of operation increases greatly with the size of runoff plots, because more sophisticated sampling and recording equipment is required. Catchment studies are desirable for studying the effects of major changes in land use, but while the measurement of large rates of runoff is fairly straightforward, it is more difficult to obtain reliable and accurate estimates of sediment movement.

Predicting soil loss

Estimating soil loss is particularly difficult, because there are so many variables, some occurring naturally, such as soil and rainfall, and also the many options for management practices. As a result, models, whether empirical or process-based, are necessarily complex if they are to include the effects of all variables. For some purposes, meaningful and useful estimates can be obtained from models, and the best example is the estimation of long-term average annual soil loss from arable land, using the Universal Soil Loss Equation (USLE). On the other hand, estimates of regional or national soil loss are of little significance or value.

The USLE will shortly be replaced by the Revised Universal Soil Loss Equation (RUSLE), which has a similar structure (that is a black-box factor empirical model), but with more sophisticated inputs and designed for operation on personal computers. After several years of development, the final draft is presently undergoing in-service testing and evaluation for publication in the near future.

In recent years, there has been a proliferation of mathematical simulation models, based on the various physical processes involved in soil detachment, transportation and deposition. Most have unrealistic titles constructed to provide an unpronounceable acronym. The complexity of the erosion process, and the need for huge data banks to compile the many algorithms which are included in the models, mean that most of this type of model require a powerful mainframe computer.


Sediment transport

Sediment movement in streams and rivers takes two forms. Suspended sediment is the finer particles which are held in suspension by the eddy currents in the flowing stream, and which settle out only when the stream velocity decreases, such as when the stream gradient flattens or the stream discharges into a pond or lake. Larger particles are rolled along the streambed, and form the bedload.

The relative quantities moved in suspension and in bedload vary greatly. At one extreme, where the sediment is coming from a fine-grained soil such as a wind-deposited loess, or an alluvial clay, the sediment may be almost entirely in suspension. On the other hand, a fast-flowing clear mountain stream may have negligible amounts of suspended matter and almost all of the movement by rolling gravel, pebbles and stones on the streambed.

The estimation of suspended load by sampling is relatively simple, but taking a representative sample of bedload is difficult. There are several sources of error associated with trying to correlate the amount of sediment measured in streams with the extent of erosion in the catchment.

Measuring sediment

Methods of measurement include the use of grab samplers (where turbulence mixes the sediment thoroughly), depth integrating samplers, point integrating samplers and pumping samplers, all of which provide a series of observations in a vertical profile. Continuous sampling is necessary when it is required to study the pattern of rising and falling rates of flow and the variations of sediment concentration at different flow rates.

Measuring the total amount of sediment deposited in ponds or reservoirs avoids the issue of the sediment delivery ratio, but unless the reservoir is large enough to contain the whole of the runoff, some of the sediment will be carried out over the spillway.

Predicting sediment movement

The difficulty of obtaining measurements of bedload has led to attempts to calculate it from more easily measured parameters, but these methods are not widely used. A simple method based on knowing the suspended sediment concentration and the texture of both suspended material and bed material is known as Maddock's Classification. There are many other formulas, and much debate on their accuracy and reliability.

Models designed for the prediction of soil loss are often concerned primarily with the loss of soil from agricultural land, but some of them may be extended to estimate sediment movement in catchments. An example is ANSWERS, (Areal Non-Point Source Watershed Environment Response Simulation), where the catchment is divided into elements where input parameters can be assumed uniform, and each small element (usually about 4 ha) is analysed separately with three models - hydrological, sediment detachment/transport, and routing components which describe the movement of water in overland, subsurface and channel flow phases. There are also other models which can be extended to catchment studies.

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