RUNOFF AND EROSION IN INTERCANOPY ZONES OF PINYON-JUNIPER WOODLANDS

In semiarid pinyon-juniper environments, the principal mechanisms of r edistribution of water, sediments, nutrients, and contaminants are run off and erosion. To study the phenomena underlying these mechanisms, w e established six 30-m2 plots, in intercanopy zones, for monitoring ov er a 2-yr period (1991-1993). Two of the plots were severely disturbed ; 4 were undisturbed. We measured the most runoff from these plots dur ing mid summer (generated by intense thunderstorms) and late winter (f rom snowmelt and/or rain-on-snow). Runoff accounted for 10 to 28% of t he water budget over the 2-yr period-a higher proportion than that obs erved in most other pinyon-juniper woodlands, which is probably explai ned by the smaller scale as well as the higher elevation of our study area. Runoff accounted for 16% of the summer water budget the first ye ar, with above-average precipitation (and thereby higher soil moisture content) and 3% the second year, when precipitation was about average . Winter runoff was substantial both years as measured on the small sc ale of our study (no winter runoff was observed in the nearby stream c hannel). Interestingly, even though precipitation was lower the first winter, runoff was higher. This may be because snowmelt set in about 2 0 days earlier that year-while the soils were still thoroughly frozen, inhibiting infiltration. Differences between disturbed and undisturbe d plots were most evident in the summer: both runoff and erosion were substantially higher from the disturbed plots. On the basis of our obs ervations during this study, we suggest that the following hypotheses proposed about runoff and erosion in other semiarid landscapes are als o true of pinyon-juniper woodlands: (1) Runoff amounts vary with scale : runoff decreases as the size of the contributing area increases and provides more opportunities for infiltration. (2) The infiltration cap acity of soils is dynamic; it is closely tied to soil moisture content and/or soil frost conditions and is a major determinant of runoff amo unts. (3) Soil erodibility follows an annual cycle; it is highest at t he end of the freeze-thaw period of late winter and lowest at the end of the summer rainy season, when soils have been compacted by repeated rainfall.