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close this bookHandbook for Agrohydrology (NRI)
close this folderChapter 6: Catchment characteristics
View the document(introduction...)
View the document6.1 Natural vegetation
View the document6.2 Interception
View the document6.3 Catchment size, slope and topography
View the document6.4 Field orientation
View the document6.5 Antecedent soil moisture conditions
View the document6.6 Other catchment influences
View the documentEquipment costs

6.5 Antecedent soil moisture conditions

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.