London scoping - ukcip

Final Report
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Appendix C
Representing Soil Moisture Variations Using
the Hydrological Model CATCHMOD
In CATCHMOD a ‘direct percolation’ mechanism, allows fixed proportions (Dp) of incoming
precipitation, that exceed the potential evaporation rate, to bypass the soil store (even during
periods of soil moisture deficit). This process represents the observed behaviour of fractured
soils and macropores during summer rainfall and is only relevant to soils overlying permeable
strata. The soil moisture sub-model is based on Penman’s (1949) drying curve such that when
the supply of moisture is limited, evaporation occurs at a constant proportion, k, of the potential
rate. The value of the soil moisture deficit above which evaporation occurs at the reduced rate,
Dc, (termed the potential drying constant) is derived via parameter optimisation. The ‘upper’
soil horizon, therefore, has a finite capacity equal to this constant. The ‘lower’ horizon is
depleted by the reduced rate only when the upper horizon is empty, and can accumulate large
deficits (as witnessed during the severe 1976 UK drought). During recharge, wetting by
precipitation fills the upper reservoir before any replenishment of the lower. When a basin zone
becomes saturated excess moisture from the soil store contributes to total percolation. Where a
soil is underlain by permeable geological formations, excess water from the overlying soil zone
percolates through the unsaturated zone to the aquifer below.