Soil water dynamics and ecosystem water balance in the semiarid West

Background/Question/Methods

Although arid and semiarid regions are defined by low precipitation, the seasonal timing of precipitation can have a profound influence ecosystem structure and functioning. The influence of precipitation seasonality is illustrated by the two large semiarid areas of the western U. S. – the Great Plains, a region that receives primarily summer precipitation and is dominated by perennial grasses, and the intermountain zone, a region that receives important winter precipitation and is dominated by woody plants.  Although these general relationships between precipitation seasonality and plant functional type are well recognized, the impact of precipitation seasonality on water cycling and ecosystem water balance is not well characterized.  Comparing water cycling and balance in the Great Plains with the intermountain zone provides an ideal framework for assessing this impact.  We used a soil water simulation model to characterize differences in soil water dynamics and ecosystem water balance between sites from the Great Plains and the intermountain zone. We selected 18 sites from each region covering a similar range of mean annual temperatures and precipitation. 

Results/Conclusions

The annual cycles of soil water potentials are very generally similar between the two regions, especially in the 0-30 cm soil layer. In the Great Plains, the time of wettest soils is between March and June, whereas in the Intermountain zone, the wet soil period begins in December and persists into late March and early April. Great Plains soils tend to be driest in late summer although winter soils also tend to be dry. This pattern becomes even clearer with depth. Intermountain soils have a clear and consistent pattern of being dry from June through September at all depths, although the dry period shifts to slightly later in the year with depth. In the Great Plains, the variability in soil water potential at the daily scale stays the same or increases with depth. By contrast, in the Intermountain zone variability in soil water potential at the daily scale decreases with depth. This result suggests that soil water at depth is a more reliable resource in the Intermountain zone than it is in the Great Plains. This may be an explanation for the greater success of deep-rooted woody plants in the Intermountain zone than in the Great Plains.