We compared three different soil water models to evaluate the extent t
o which variation in plant growth form and cover and soil texture alon
g a topographic gradient interact to affect relative rates of evaporat
ion and transpiration under semiarid conditions. The models all incorp
orated one-dimensional distribution of water in the soil and had separ
ate functions for loss of water through transpiration and soil evapora
tion but differed in the degree of mechanism and emphasis. PALS-SW Pat
ch Arid Lands Simulator-Soil Water) is a mechanistic model that includ
es soil water fluxes and emphasizes; the physiological control of wate
r loss by different plant life forms along the gradient. 2DSOIL is a m
echanistic model that emphasizes the physical aspects of soil water fl
uxes. SWB (Soil Water Budget) is a simple water budget model that has
no soil water redistribution and includes simplified schemes for soil
evaporation and transpiration by different life forms. The model predi
ctions were compared to observed soil water distributions at five posi
tions along the gradient. All models predicted soil water distribution
s reasonably well and, for the most part, predicted similar trends alo
ng the transect in the fractions of water lost as soil evaporation ver
sus transpiration. Transpiration was lowest (about 40% of total evapot
ranspiration (ET)) for the creosote bush community, which had the lowe
st plant cover (30% peak cover). The fraction of ET as transpiration i
ncreased with increasing plant cover, with 2DSOIL predicting the highe
st transpiration (60% of total ET) for the mixed vegetation community
(60% peak cover) on relatively fine textured soil and PALS-SW predicti
ng highest transpiration (69% of total ET) for the mixed vegetation co
mmunity (70% peak cover) on relatively coarse textured soil. The commu
nity type had an effect on the amount of water lost as transpiration p
rimarily via depth and distribution of roots. In this respect, PALS-SW
predicted greatest differences among stations as related to differenc
es in plant community types. However, since FALS,SW did not provide as
good of fit with the soil moisture data as did 2DSOIL, the difference
s in the morphology and physiology of the life-forms may be secondary
to the overall control of water loss by the primary factors accounted
for in 2DSOIL: vertical distribution of soil moisture. degree of canop
y cover, and evaporative energy budget of the canopy. Soil texture int
eracted with the amount and type of plant cover to affect evaporation
and transpiration, but the effect was relatively minor.