Water Quantity

There has been much interest in using vegetation management to increase water yields (runoff and percolation) from rangeland and forest watersheds in the southwestern United States. An option often considered [Hibbert, 1983; Carlson et al., 1990; Jofre and Rambal, 1993; Davis, 1993] is to replace deep-rooted woody species, which may intercept a substantial amount of precipitation [Eddleman and Miller, 1991] and have high whole-plant transpiration rates due to high leaf areas [Angell and Miller, 1994; Owens, 1996], with shallow-rooted herbaceous vegetation that usually intercepts less precipitation and has less leaf area. The amount of increased water yields from these watersheds, if any, resulting from vegetation management depends upon vegetation type or land use [Dunn and Mackay, 1995], vegetation treatment type or soils [Richardson et al., 1979], and climate [Griffen and McCarl, 1989].

Other field research in Edwards and Uvalde counties indicates that brush control and grazing management can increase water yields (Table 1). The amount of increased water yields depends upon site and environmental conditions. Field research (Table 1) was conducted on small areas, ranging from 1 to 15 ha, and involved a number of soil types, rainfall regimes, and research methods.

Dugas et al. (1998) showed that daily ET from the area to be treated was 0.05 mm/d lower than that from the untreated area during the two-year pre-treatment period, while it was 0.12 mm/d lower during the three-year post-treatment period. The ET difference (untreated minus treated) was 0.3 mm/d in the first two years following removal of J. ashei, and decreased thereafter. Removal of J. ashei had no consistent effect on runoff. Vegetation management increased the potential for greater water yields in the short term from these rangelands by decreasing ET for the first two years after imposition of treatment.

Table 1. Water savings (per unit area of treated land) from land use changes, primarily removal of ashe juniper, at selected locations around and on the Edwards Plateau. Water savings at locations with a single asterisk are based on model simulations.

Location Reference Land Use Change Water Savings(m3/ha/yr)

Sonora Thurow and Taylor (1995) 60% Juniper/40% grass to 100% grass 940

Annandale Owens and Knight (1992) Removal all Juniper 1250**

Seco Ck. Dugas et al. (1998) Removal all Juniper (1st year after treatment) 980

Seco Ck. Wright (1996) Remove 70% of Juniper (1st 14 months after treatment. Adjusted for reduced precipitation) 1125

N. Concho* Upper Colo. R. Auth. (1998) Remove all Brush (Mesquite and Juniper) 280

**Calculated from ratio of average runoff to precipitation and from measured increase in runoff.

December 17, 2002
Dennis Hoffman, Project Leader