Effects of long-term rainfall variability on evapotranspiration and soil water distribution in the Chihuahuan Desert: A modeling analysis

We used the patch arid land simulator (PALS-FT) - a simple, mechanistic eco system model - to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: a nnuals, perennial forbs, C-4 grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenolo gical activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plan t community types (grass- vs. shrub-dominated) were carried out for the Jor nada Basin, New Mexico, using 100 years of daily precipitation data (1891-1 990). In order to emphasize variability associated with rainfall and fundam ental differences in FT composition between communities, the seasonal patte rns cover of perennials were held constant from year to year. Because the r elative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model pr edictions of ET by allowing their cover to be variable, fixed, or absent. Over the entire 100-yr period, total annual ET is highly correlated with to tal annual rainfall in both community types, although T and E alone are les s strongly correlated with rainfall, and variation in transpiration is near ly 3 times greater than evaporation and 2 times greater than variation in r ainfall (CV of rainfall = 35%). Water use shows a relatively high similarit y between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year compari son between communities, T/ET was significantly greater in the grass-domina ted community, reflecting the fact that over the long term more than half o f the rain occurs in the summer and is used slightly more efficiently (T ?E ) by the C-4-grass community than the shrub community, although we found so me rainfall patterns that resulted in much greater T/ET in the shrub commun ity in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulatio n, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub -dominated site). These predictions suggest that because of the relatively shallow distributi on of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular 'windows' of moisture availability via differences in phenologies. We found very little differenc es in average, long-term model predictions of T, E, and ET when annual plan t cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studie s about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variabil ity that is possible.