A comparative modeling study of soil water dynamics in a desert ecosystem

Abstract

Abstract. We compared three different soil water models to evaluate t. he extent to which variation in plant growth form and cover and soil texture along a topographic gradient interact to affect relative rates of evaporation and transpiration under semiarid conditions. The models all incorporated one-dimensional distribution of water in the soil and had separate functions for loss. of water through transpiration and soil evaporation but differed in. the degree of mechanism and emphasis. PALS-SW (Patch Arid Lands Simulator-Soil Water) is a mechanistic model that includes soil water fluxes and emphasizes the physiological control of water loss by different plant life forms along the gradient. 2DSOIL is a mechanistic model that emphasizes the physical aspects of soil water fluxes. 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 predictions were compared to observed soil water distributions at five positions along the gradient. All models predicted soil water distributions reasonably well and, for the most part, predicted similar trends along the transect in the fractions of water lost as soil evaporation versus transpiration. Transpiration was low.est (about 40% of total evapotranspiration (ET)) for the creosote bush community, which had the lowest plant cover (30% peak cover). The fraction of ET as transpiration increased with increasing plant cover, with 2DSOIL predictin. g the highest transpiration (60% of total ET) for the mixed vegetation community (60% peak cover) on relatively fine textured soil and PALS-SW predicting highest transpiration (69% of total ET) for the mixed vegetation community (70% peak cover) on relatively coarse textured soil. The community type had an effect on the amount of water lost as transpiration primarily via depth and di. stribution of roots. In this respect, PALS-SW predicted greatest differences amon. g stations as related to differences in plant community types. However, since PALS-SW did not provide as good of fit with the soil moisture data as did 2DSOIL, the differences 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 canopy cover, and evaporative energy budget of the canopy. Soil texture interacted with the amount and type of plant cover to affect evaporation and transpiration, but the effect was relatively minor.

Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications

“…As discussed above, ecosystems in semi-arid regions are highly coupled morphological-ecological-hydrological systems and water is the key factor in determining plant growth (Walker et al, 1981;Ehleringer et al, 1991;Kemp et al, 1997;Rodriguez-Iturbe et al, 1999). The arrangement of vegetation in stippled, labyrinth or banded patterns is obviously an efficient way to trap most of the local precipitation and the water that runs off from the upslope bare soil patches on to the vegetated patches.…”

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

“…Following several rain events, Dugas et al (1996) found that average T/ET ranged from 40-70% over several different Chihuahuan shrub sites in New Mexico. Kemp et al (1997) simulated the annual ET partitioning of several different Chihuahuan Desert communities and found that annual T/ET was about 40% for a creosote community with 30% peak cover, but T/ET increased to 60% in a mixed community with about 60% cover. Again, we expect that seasonal T/ET is highly variable even at one site because of climate variability and differential plant functional responses (Reynolds et al, 2000).…”

Modeling multiyear observations of soil moisture recharge in the semiarid American Southwest

“…Andraski [1997] found that the maximum depth to which temporal changes in water content were observed over 5 year period was 0.75 m under vegetated soil. Kemp et al [1997] performed a comparative modeling study of soil water dynamics in a desert ecosystem and concluded that over the course of their 1 year study, rainfall recharge penetrated to a depth of 0.45-0.55 m. Cable [1980] reported that water infiltrated past the 0.25 m depth eight times and to a depth of at least 1.0 m only three times in a 3 year study in southern Arizona.…”