Technical InformationThe infiltration module of LAPSUS is elaborated to improve representation of spatial diversity in water redistribution and availability within arid catchments. In this elaborated version of LAPSUS infiltration I (m time-1) is subtracted of all available water in a grid cell, including run-on, where in the original LAPSUS model infiltration is subtracted of the precipitation only. Infiltration I is dependent on the available water in a grid cell by the following relationship:
(1.1) I = If · Q
Discharge Q (m time-1) is a summation of precipitation on a grid cell plus run-on, whereby evapotranspiration and drainage are assumed zero. This assumption is applicable in the region for which this module was programmed as Yair and Lavee (1985) demonstrated for Halluqim that during a storm, when cloudiness is high and air temperature low, the potential evapotranspiration is insignificant and has no effect on the rainfall-runoff relationship. Subsurface drainage is set to zero as well, as this module was only used modelling single events, and therefore longer term processes as drainage are ignored. The infiltration procedure has the advantage that it calculates infiltration only when total water on a grid cell (precipitation and runon) is known, which closely resembles the natural processes during a precipitation event.
Surface cover dependence:LAPSUS is further adapted to handle the influence of different surface covers, whereby the infiltration fraction If (-) is dependent on vegetation cover V (%), as follows:
(1.2) If = Iv ∙ (V / 100) + Ib ∙ (1- (V / 100))
whereby Iv is the fraction of Q infiltrating under vegetation and Ib is the infiltration fraction for crust covered surface. This vegetation-dependent infiltration algorithm represents Hortonian overland flow processes in the model, as it differentiates between vegetated areas with a high infiltration rate and crusted areas with a low infiltration rate. Spatial variability in water redistribution is additionally influenced by water storage capacity of the soil, which regulates the saturation overland flow. Infiltration is corrected for available storage capacity per grid cell, which equals soil depth times pore volume (Ø in m3 m-3) of the soil, giving the maximum possible infiltration volume (m time-1).
As vegetation has a different sensitivity to incision and deposition than bare surface, different values for Kes and Pes are used for bare and vegetated surfaces (Calvo-Cases, 2003).
- Yair, A. and Lavee, H., 1985. Runoff generation in arid and semi-arid zones. In: M.G. Anderson and T.P. Burt (Editors), Hydrological Forecasting. John Wiley & Sons Ltd., pp. 183-220
- Calvo-Cases, A., Boix-Fayos, C. and Imeson, A.C., 2003. Runoff generation, sediment movement and soil water behaviour on calcareous (limestone) slopes of some Mediterranean environments in southeast Spain. Geomorphology, 50(1-3): 269-291.