PARENT SESSION
Organized Oral Session 41: Ecological responses to precipitation: Scaling patterns and processes from the genome to the ecosystem
Organizer(s): ME Loik and SD Smith
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

The temporal and spatial distributions of precipitation: Ecological and ecohydrological significance.

Lauenroth, William^*,1, Bradford, John², ¹ Colorado State University, Fort Collins, CO, USA² USDA Forest Service Rock Mountain Station, Fort Collins, CO, USA

ABSTRACT- Two characteristics of arid and semiarid regions are that evaporative demand of the atmosphere is high and relatively constant in space and time, whereas precipitation is low and variable in space and time. Both have substantial consequences for ecosystem water budgets. Two important characteristics that are less well understood are 1) how daily precipitation amounts are distributed across size classes and 2) how the duration of dry intervals between daily events are distributed across length classes. Whether precipitation is received in mostly small or large events and whether the intervals between events are short or long can have huge consequences for water budgets and the activities of the biota. To characterize distributions of event sizes and dry period lengths, we analyzed 30-year time series from 800 weather stations in the arid and semiarid areas of the central and western U.S. between 90 and 120 W longitude. There is an enormous range in the importance of the smallest (<=5 mm) and the largest (>30 mm) events as well as the shortest (<=10 days) and longest (>30 days) inter-event intervals. Sites with the largest proportion of small events (>0.7) were in the Great Plains and those with the smallest (<0.25) in the intermountain zone. The opposite was true for large events. The greatest proportion of long inter-event intervals (>0.35) was found in the intermountain zone and the greatest proportion of short intervals (>0.95) in the Great Plains. To evaluate the ecological and ecohydrologic consequences of these differences among sites, we used a daily time step soil water simulation model to estimate daily evaporation and transpiration. In dry areas, sites with a predominance of small events tended to have greater evaporation compared to transpiration than sites with primarily large events. For all sites, short intervals between events tended to maximize evaporation losses compared to long intervals. These results imply higher ecosystem water use efficiency in areas with large events and long dry intervals and provide insight into how the size and frequency of precipitation events influence arid and semiarid ecosystems. Recognizing the importance of precipitation event size and inter-event length distributions will help us understand important ecological differences across the semiarid and arid regions of the U.S.

Key words: ecohydrology, water budget, semiarid, arid