Changes in Streamflow in an Herbicide-Treated Jumpier Watershed in Arizona

Abstract

A 147-ha juniper watershed in north -central Arizona was sprayed with an herbicide mixture to kill all overstory vegetation. After the area was sprayed, annual water yield increased significantly when flow was greater than 12 mm. The ratio of mean annual quick flow to event flow prior to treatment was 0.86 and remained essentially the same after treatment. The herbicide treatment reduced evapotranspiration losses and increased water yield by killing the overstory trees and leaving them in place. These dead trees provided some shade and wind resistance and created a microclimate that reduced evaporation and enabled the soil below 30 cm to remain above its soil moisture wilting point. Although mean annual water yield increased by 27% (6 mm, 8-year mean), this increase may not be practical from a management view point. Therefore, it is unlikely that extensive juniper acreage will be treated. The amount of treated acreage will depend on the demand for water and on the value of water in the market place. The area treated will also be constrained by consideration of other resource values and desires of the public.