MAPPING SUSCEPTIBILITY OF SAND DUNES TO DESTABILIZATION ON THE NAVAJO NATION, SOUTHERN COLORADO PLATEAU

Sand dunes cover approximately one-third of the arid to semi-arid 71,000 km2 Navajo Nation on the southern Colorado Plateau. Reactivation of stabilized sand is occurring in many areas of the Navajo Nation, greatly impacting grazing and farming resources, living conditions, and infrastructure. The degree of dune mobility can be expressed as a function of climate variables, which include the percentage of time that wind is capable of moving sand and the balance between precipitation (P) and potential evapotranspiration (PE). Winds capable of transporting sand occur frequently in this region. Therefore, the ratio P/PE has been shown to be a critical factor controlling dune mobility because of its direct link to the amount of stabilizing vegetation. We mapped the distribution of normal average ranges of P/PE, calculated from all the years of record for each station. The mapped distribution of eolian deposits are shown on a second layer in GIS. Based on these calculations, and the known climatic variability of the past 100 years, sand dunes on the Navajo Nation exist under meteorological climatic conditions that promote the entire spectrum of dune mobility, from mostly stable (during humid periods), mostly active, to fully active (during periods of drought). Although the Navajo Nation has an overall moisture deficit (P/PE < 1.0), enough moisture is present to support some stabilizing vegetation under “normal average” conditions, albeit marginally in some areas. Given the current severe drought conditions, however, P/PE values have been much lower than average. Therefore, we expect dunes to become active in areas that have low P/PE, over the period of instrumental record. Thresholds in P/PE for changes in dune mobility are based on observations by previous workers, with transitions from mostly stable to mostly active at P/PE=0.315, and from mostly active to fully active at P/PE=0.125.