Assessing and monitoring the health of western rangeland watersheds

The most important function of watersheds in the western U.S. is the capaci ty to retain soil and water, thereby providing stability in hydrologic head and minimizing stream sediment loads. Long-term soil and water retention v aries directly with vegetation cover. Data on ground cover and plant specie s composition were collected from 129 sites in the Rio Grande drainage of s outh-central New Mexico. This area was previously assessed by classificatio n of Advanced Very High Resolution Radiometry (AVHRR) imagery. The classifi cation of irreversibly degraded sites failed to identify most of the severe ly degraded sites based on size of bare patches and 35% of the sites classi fied as degraded were healthy based on mean bare patch size and vegetation cover. Previous research showed that an index of unvegetated soil (bare pat ch size and percent of ground without vegetative cover) was the most robust indicator of the soil and water retention function. Although the regressio n of mean bare patch size on percent bare ground was significant (p < 0.001 ), percent bare ground accounted for only 11% of the variability in bare pa tch size. Therefore bare patch size cannot be estimated from data on percen t bare ground derived from remote sensing. At sires with less than 25% gras s cover, and on sites with more than 15% shrub cover, there were significan t relationships between percent bare soil and mean bare parch size (p < 0.0 5). Several other indicators of ecosystem health were related to mean bare patch size: perennial plant species richness (r = 0.6, p < 0.0001), percent cover of increaser species (r = 0.5, p < 0.0001) and percent cover of fora ge useable by livestock (r = 0.62, p < 0.0001). There was no relationship b etween bare patch size and cover of species that are toxic to livestock. In order to assess the ability of western rangeland watersheds to retain soil and water using remote sensing, it will be necessary to detect and estimat e sizes of bare patches ranging between at least 0.5 m in diameter to sever al meters in diameter.