Precipitation patterns and
soil moisture dynamic

In arid ecosystems, moisture in deeper soil layers is replenished primarily by winter precipitation. The colder temperatures and reduced evaporative is necessary to allow soil water to accumulate and infiltrate into deeper layers. Once infiltrated beyond a certain depth (approximately 20- 30 cm), it is not subject to evaporative losses and is depleted only by plant uptake. Thus, water from winter precipitation remains available within the soil into summer and is used simultaneously with water from summer rainfall events. We are interested to find out how important warm-season rainfall is to plant growth compared to stored water in deeper soil layers left there by from winter precipitation.
To address this question, we must first identify where, how much and for how long water from summer rainfall events is available to desert plants. Rainfall records from our field site on the Colorado Plateau show that about as much water falls during the cold season as during the warm season. A frequency distribution for rainfall events shows that most events are small (Schwinning and Ehleringer, in review). Furthermore, a large fraction of total summer precipitation is deposited as rainfall of 12 mm or less. Observations from our field sites have shown that events of this size typically wet the soil to no more than 10 cm below the soil surface and for less than 10 days. Thus, to exploit this source of water, plants must have active roots very near the soil surface and they must be capable of extracting soil moisture fast, before it is lost to soil evaporation (Schwinning et al. 2001).

Related Publications

  • Schwinning, S., Davis, K., Richardson, L. and Ehleringer, J.R. 2001. Water pulse use of three Colorado Plateau plant species. Oecologia, in press.

  • Schwinning, S. and Ehleringer, J.R. Rainfall size effects on the water status and water use of Colorado Plateau plants. Oecologia, in review