Earth Surface Processes and Landforms Earth Surf. Process. Landforms 27, 1011–1020 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.389 VEGETATION INFLUENCES ON WATER YIELDS FROM GRASSLAND AND SHRUBLAND ECOSYSTEMS IN THE CHIHUAHUAN DESERT MEL NEAVE 1 * AND ATHOL D. ABRAHAMS 2 1 Division of Geography, Madsen Building (F09), University of Sydney, NSW 2006, Australia 2 Department of Geography, State University of New York at Buffalo, Buffalo, NY 14261, USA Received 18 March 2001; Revised 19 December 2001; Accepted 16 January 2001 ABSTRACT This study examines runoff generated under simulated rainfall on Summerford bajada in the Jornada Basin, New Mexico, USA. Forty-five simulation experiments were conducted on 1 m 2 and 2 m 2 runoff plots on grassland, degraded grassland, shrub and intershrub environments located in grassland and shrubland communities. Average hydrographs generated for each environment show that runoff originates earlier on the vegetated plots than on the unvegetated plots. This early generation of runoff is attributed to soil infiltration rates being overwhelmed by the rapid concentration of water at the base of plants by stemflow. Hydrographs from the degraded grassland and intershrub plots rise continuously throughout the 30 min simulation events indicating that these plots do not achieve equilibrium runoff. This continuously rising form is attributed to the progressive development of raindrop-induced surface seals. Most grassland and shrub plots level out after the initial early rise indicating equilibrium runoff is achieved. Some shrub plots, however, display a decline in discharge after the early rise. The delayed infiltration of water into macropores beneath shrubs with vegetation in their understories is proposed to explain this declining form. Water yields predicted at the community level indicate that the shrubland sheds 150 per cent more water for a given storm event than the grassland. Copyright  2002 John Wiley & Sons, Ltd. KEY WORDS: water yield; semiarid runoff; simulated rainfall INTRODUCTION Surveys undertaken in the Jornada del Muerto Basin in southern New Mexico, USA, indicate that in 1858 the region was extensively covered by semiarid grasses (Buffington and Herbel, 1965). Today, however, a variety of shrub species, such as creosotebush (Larrea tridentata ), mesquite (Prosopis glandulosa ), and tarbush (Flourensia cernua ), dominate the basin flora (ARS, 1994). The causes of this change in vegetation are complex and intricate; some that have been proposed include climatic change, fire suppression, increasing atmospheric CO 2 , and overgrazing (Neilson, 1986; Grover and Musick, 1990; Milchunas and Laurenroth, 1993; W. H. Schlesinger, unpublished work). Whatever the factors responsible for the initial disruption of the grasslands, the consequence has been the growth of apparently self-perpetuating shrubland ecosystems which appear to be very resilient to environmental disruption or change and therefore extremely persistent. One important aspect of the invasion of shrubs in the Jornada Basin has been a redistribution of resources. Semiarid grasslands afford soils a relatively complete and homogeneous cover. Semiarid shrubland communi- ties, however, typically display a heterogeneous distribution of vegetation. The shrubs become focal points, or ‘islands of fertility’, beneath which soil infiltration rates are relatively high and nutrients accumulate and are cycled. At the same time, adjacent intershrub areas become hostile zones of exposed soils and low nutrient availability (W. H. Schlesinger, unpublished work). The focus of the Jornada Long-Term Ecological Research (LTER) programme is on the processes that have led to the desertification of the northern Chihuahuan Desert ecosystems in the Jornada Basin and on the concomitant changes in ecosystem structure (Whitford, 1993; W. H. Schlesinger, unpublished work). This * Correspondence to: M. Neave, Division of Geography, Madsen Building (F09), University of Sydney, NSW 2006, Australia. E-mail: mneave@geography.usyd.edu.au Copyright  2002 John Wiley & Sons, Ltd.