Soil moisture deficit

The calculation of potential evaporation (Ep) from readily available meteorological data is seen to be a much simpler operation than the computation or measurement of actual evapotranspiration (Et) from a vegetated surface. However, water loss from a catchment area does not always proceed at the potential rate, since this is dependent on a continuous water supply. When the vegetation is unable to abstract water from the soil, then the actual evaporation becomes less than potential. Thus the relationship between Et and Ep depends upon the soil moisture content. Early attempts to quantify actual evapotranspiration and crop water use by agricultural hydrologists made use of the concepts of soil moisture deficit and field capacity in defining this relationship.

When the soil is saturated, it can hold no more water. After rainfall ceases, saturated soil relinquishes water and becomes unsaturated until it can just hold a certain amount against the forces of gravity; it is then said to be at 'field capacity'. A consideration of the principles of soil physics suggests that this is not a fixed capacity, but practically after a day or two of drainage, hydraulic conductivities and rates of gravity drainage in most soils become rather slow so that further drying will be affected as much by evapotranspiration as by drainage. Under these conditions, the negative pressure potential may be approximately —100 hPa (see Section 6.3).

When the soil is close to field capacity, Et = Ep, evapotranspiration occurs at the maximum possible rate determined by the meteorological conditions. If there is no rain to replenish the water supply, the soil moisture gradually becomes depleted by the demands of the vegetation to produce a soil moisture deficit (SMD), defined as the amount of water required to restore the soil to field capacity. As SMD increases, Et becomes increasingly less than Ep. The values of SMD and Et vary with soil type and vegetation, and the relative changes in Et with increasing SMD have been the subject of considerable study by botanists and soil physicists. Penman (1950b) introduced the concept of a 'root constant (RC) that defines the amount of soil moisture (millimetre depth) that can be extracted from a soil without difficulty by a given vegetation. Thereafter, Et becomes less than Ep as moisture is extracted with greater difficulty as shown in Fig. 10.11. As the SMD increases further, the vegetation wilts and Et becomes very small or negligible. Before the onset of wilting, vegetation will recover if the soil moisture is replenished, but there is a maximum SMD for each plant type at a 'permanent wilting point' (typically —15 000hPa) from which the vegetation cannot recover and dies. The total water available to the vegetation is called the available water capacity (AWC).

There have been a variety of SMD models of this type reported in the literature. One of the most interesting is a comparison of different formulations by Calder etal. (1983). Fig. 10.12 shows the sequence of SMD predicted for two sites in the UK in comparison with measured SMDs based on neutron probe soil moisture profile measurements. In both cases, the best fitted models are shown. It is clear that these simple concepts can reproduce the observations quite well, especially when it is remembered that, for the Thetford Forest site, the years 1975/1976 were among the driest on record. In fact, this

Fig. 10.11 Representative decline of Et/Ep with soil moisture deficit (storage less than field capacity).

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Soil moisture deficit (mm)

Fig. 10.11 Representative decline of Et/Ep with soil moisture deficit (storage less than field capacity).

study also showed that the very simplest approaches could do well. One of the model structures that they tried used only a one parameter annual sine wave to estimate daily potential evapotranspiration such that:

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