Within the United States, approximately 70% of Department of Defense lands are located in arid and semi-arid environments. The overall majority of this land that is suitable for wheeled or tracked vehicles is desert piedmont plains (bajadas) largely comprised of alluvial fans covered with desert (rock) pavement and active washes. The greatest concentration of biomass and biodiversity in these areas is associated with vegetation along rills and tributary channels that drain alluvial fans as well as along main alluvial washes. This distribution of vegetation is primarily the result of a considerable increase in available water that is derived from the ephemeral runoff of precipitation generated from surrounding soils and desert pavements. In some cases, this runoff-supplied water may be two or three times the amount of water received from precipitation alone. Any change in the runoff water, whether the result of military activities or natural environmental change, directly impacts the ecosystem.
The overall goal of this SERDP Exploratory Development (SEED) project was to investigate whether specific shrub and tree species distributed along the first-order channels draining desert piedmonts can provide efficient and reliable signals of environmental change (resulting from either natural disturbance or military activities) and ecological condition. Specific objectives were to: (1) determine the historic range in variation of selected desert vegetation common to alluvial fan surfaces and first-order channels; (2) evaluate if changes in soil and surface hydrology, resulting from either military activities or natural environmental variation, could be shown to predominantly account for changes in ecosystem condition; and (3) provide recommendations for further developing and testing procedures that can be used to monitor ecosystem status and identify impacts related to natural disturbance versus military activities.
Six first-order basins were studied ranging in size from approximately 10,000–48,000 m². These basins had a nearly continuous cover of desert pavement prior to any disturbance. Natural and impacted basins were paired geographically to ensure similar soil and climatic conditions. The soil and surface cover of these basins, as well as the limitation of vegetation to drainage channels, was representative of land surface conditions in arid and hyperarid regions of the Southwest and the world. Soil and geomorphic measurements and analysis provided insights into the spatial distribution of soil moisture and the soil properties that impact soil water and vegetation. Methods and standard laboratory investigations data were provided on the volume of surface runoff per precipitation event and the extent of the hydrologic connection between surface anomalies and surface drainage. Biologic characterizations included study of species common to this area—creosotebush, ironwood, and blue paloverde.
Understanding physical and biological signals that indicate change in the flux and availability of water because of either military or environmental causes will improve the management of sensitive desert lands. This project advanced the knowledge of fundamental soil-hydrology-vegetation processes that are common to desert piedmonts. Evaluation of critical linkages between soils, soil water balance, and desert plant ecology provides key information about the impact of climate and military activities on desert shrubs, soils, and archeological sites. (SEED Project Completed - 2004)