Analysis of an extreme monsoon event on the ecohydrologic and geomorphologic conditions in a semiarid basin based on field instrumentation and reconnaissance

A fundamental problem in hydrologic science lies in understanding the interrelation among soil development, water and energy fluxes, and the mediating effect of vegetation in regulating the nature and extent of these processes. A semiarid basin in central New Mexico composed of three distinctive aspect slopes has been instrumented to study the ecohydrologic dynamics affecting pedogenic and geomorphic processes. Aspect differences give rise to distinct soil and plant community characteristics. The north-facing slope is composed of a juniper-grass association with fine textured soils and organic matter, while the south facing slope consists of a creosote-grass community with coarser soils and shallow calcium carbonate horizons. Between these lies an east-facing headslope serving as an ecotone. In this study, we describe the basin characteristics along with ongoing data collection efforts in the basin. Furthermore, an analysis of the effect of an extreme flood event and its ecologic, hydrologic and geomorphic implications in the area is presented. This recent event led to observable geomorphic changes in the watershed due to overland flow and channel discharge, including rill incision and arroyo widening. A series of rain gauges deployed in each slope show differences in the distribution and incidence of rainfall over the basin. Observations from soil moisture probes placed along transects at canopy and intercanopy sites in the different slopes provide evidence of the effect of soil and vegetation cover on moisture fluxes during the flood event. The analysis is supported by field observations documented before and after the summer monsoon flood. For example, calculations of peak discharge were based on post-event measurements of the water stage along the channel indicated by fallen vegetation. High precision GPS was also used to obtain relevant data about channel and topographic characteristics of the area. The short-term flood event analysis provides insights into terrain-soil-vegetation feedbacks acting on the availability and movement of water and sediments through the system.