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Rainfall, soil moisture and runoff dynamics in New Mexico piñon–juniper woodland watersheds

TitleRainfall, soil moisture and runoff dynamics in New Mexico piñon–juniper woodland watersheds
Publication TypeReport
Year of Publication2005
AuthorsOchoa, CG, Fernald, A, Tidwell, VC
Series TitleEcology, Management and Restoration of Pinon-Juniper and Ponderosa Pine Ecosystems: Combined Proceedings of the 2005 St. George, Utah and 2006 Albuquerque, New Mexico Workshops
Date Published01/2008
InstitutionU.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station
Keywordsbaseline data, New Mexico, piñón-juniper, rainfall, runoff, soil moisture, TDR, wetting depth

Clearing trees in piñón-juniper woodlands may increase grass cover and infiltration, leading to reduced surface runoff and erosion. This study was conducted to evaluate piñón-juniper hydrology conditions during baseline data collection in a paired watershed study. We instrumented six 1.0 to 1.3 ha experimental watersheds near Santa Fe, NM to collect rainfall, soil moisture, and runoff data. Volumetric water content reflectometers (VWCRs) were used to measure soil moisture on wooded hillslopes, on grass hillslopes, and in valley bottoms. Time domain reflectometry (TDR) probes were installed in two watersheds to measure soil wetting at hillslope, gully headcut and channel locations. Spatial dynamics of rainfall and runoff interactions were evaluated for 9 rainfall events. Runoff was present at all watersheds in 3 of the 9 rainfall events. Rainfall intensity of 5mm/15min was generally the minimum precipitation required to generate channel runoff. During high intensity rain storms, greater soil moisture was observed at valley bottoms when compared to grass hillslopes and wooded hillslopes. At the watershed scale, total runoff as a proportion of precipitation was relatively low. Wetting depth measured with TDR probes was consistent with increases in soil moisture measured with VWCRs. In general, the wetting depth was greater at the channel, followed by the gully headcut and then by the hillslope. Results to date from the baseline data collection period suggest that larger storms that wet the entire watersheds produce most annual runoff, so clearing trees may increase grass cover, but may have little effect on annual runoff.