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A consistent encroachment of shrubs into the northern Chihuahuan Desert (NM, USA) has been well documented since the mid-1800s. This phenomenon is mainly attributed to overgrazing followed by nutrient depletion, which impedes successful grass establishment. Significantly, higher grass cover that were measured by us during 1998 and 2005 in plots at the Sevilleta National Wildlife Refuge, SEV (60.5-62%), than in plots at the Jornada Experimental Range, JER (4.7-4.9%), lead us to hypothesize that climatological factors that may affect soil moisture may account for the differences in grass cover in both sites. We carried out a 10-year analysis (1994-2004) of rain, temperature, wind speed, relative humidity, and potential evaporation at the 2 sites. Total summer rainstorm duration in JER was shorter (43 hr in comparison to 46 hr in SEV), and the temperature in JER was higher (by similar to 3 degrees C), relative humidity lower (by 3-6%) and vapor pressure deficit higher (by 0.5-0.6 kPa) in comparison to SEV. In addition, during the summer growing period, hourly wind speed was by 18.0% higher and monthly pan A evaporation was by 9.1% higher in JER, pointing out that time duration during which the soil remains wet during the summer is apparently much shorter in JER. Our results indicate that surface wetness in JER following rainstorms is apparently below the necessary threshold needed for successful grass establishment in JER. This in turn has broad consequences on the ecosystem hydrology, structure, and function.