J 5.6 PILPS SEMI-ARID EXPERIMENT: PRELIMINARY RESULTS Luis A. Bastidas* 1 , Enrique Rosero 1 , Bart Njissen 2 1 Civil and Environmental Engineering and Utah Water Research Laboratory, Utah State University, Logan, Utah 2 Hydrology and Water Resources, University of Arizona, Tucson, Arizona 1. INTRODUCTION AND SCOPE The Project for Intercomparison of Land-surface Parameterizations Schemes (PILPS) is part of the Global Land Atmosphere System Study (GLASS) whose goal is to improve the understanding of the parameterization of the interactions between the atmosphere and the continental surface [Pitman and Henderson-Sellers, 1998] though intercomparisons of current state-of-the-art parameterization schemes. GLASS is part of GEWEX, the Global Energy Water Experiment, which is a program of the World Climate Research Program (WCRP) [Lettenmmaier, 2003]. The PILPS semi-arid experiment (also known as PILPS San Pedro) has unique characteristics. It not only focuses on a different environment from previous PILPS experiments, but it also will employ appropriate system methods for parameter estimation, that will help the modeling groups in identifying parameter sets that make the models consistent with the observational data. In this preliminary work, two 4+ year long data sets from the * USDA Experimental Watershed in the Walnut Gulch, Arizona [Emmerich, 2003], are used for model intercomparison analysis with focus on evaluating the consistency the net energy partition into latent, sensible and ground heat flux components. Additionally, because the data sites correspond to two different semi-arid environments (shrub and grass) we attempt to diagnose the extent to which the standard representation of the eight participant models is adequate. 2. BACKGROUND Despite the fact that 1/3 of the global land surface is semi-arid or arid, none of the previous PILPS experiments [e.g., Njissen et al., 2003, Boone et al., 2001; Chen et al., 1997, Lettenmaier et al., 1996, Pitman et al.,1993, Henderson-Sellers et al.,1995, Henderson-Sellers et al.,1993] has focused on comparing the performance of land-surface models in such environments. In water controlled regions such as the arid U.S. Southwest, vegetation is highly dependent on imposed stresses. Its response to precipitation is distinct and variable, i.e. grass species (C 4 ) respond quickly to upper soil moisture during the summer monsoon while shrub plants (C 3 ) tend to use deeper soil moisture reserve and being active in spring and fall [Scott et al., 2000, Kemp, 1983]. The role played by the biomes as a linkage * Corresponding author address: Luis A. Bastidas, Utah Water Research Laboratory, Utah State University, Logan, UT 84322-8200; e-mail: luis.bastidas@usu.edu between soil moisture and evaporation process is especially relevant since soil moisture strongly controls the nature of water, energy and momentum fluxes, such as the partitioning of available energy between latent and sensible heat [Entekhabi and Rodriguez-Iturbe, 1994]. Predicting the availability of water resources in these hydrologically stressed regions depends fundamentally on the ability to understand and reproduce the interaction of vegetation processes with climate and its effects on the water cycle. However, many models have been working under the assumption that semi-arid areas are homogeneous (therefore, assigning similar set of parameters for all biomes) and even considering them as bare-soil areas neglecting the interaction vegetation-atmosphere [Bastidas et al., 2001, Bastidas et al., 2002]. Accurate representation within the modeling framework used for land-surface-atmosphere schemes is crucial not only for analyzing the current state of the system but also for making predictions of potential climate change impacts, assessing the effect of changes in vegetation type (i.e., shrub invasion) or increase in demand due to population growth [Hogue et al., 2005]. The unique characteristics of PILPS San Pedro allows not only to asses the ability of the models to reproduce the water and energy exchanges in semi-arid environments but also to test if the current (usually single) parametric representations of semi-arid lands in the LSM are enough to simulate the different environments [Bastidas et al., 2003]. 3. SITES, MODELS AND DATA 3.1. Lucky Hills and Kendall Sites The initial part of the experiment has been carried out at two sites within the Walnut Gulch Experimental Watershed in Southeastern Arizona, a sub-basin of the Upper San Pedro Basin. The Lucky Hills site (110°03’05’’ W, 31°44’37’’ N) is located in the lower (1372 masl) shrub dominated part of the basin. The vegetation consists mainly of the C 3 species [Scott et al., 2000]. Soils are mostly loamy sand or very gravelly sandy loams. Canopy height is estimated at 1 m. Slopes are 3-8%. Average temperature is 18.6 °C. The Kendall site (109°56’28’’ W, 31°44’10’’ N) is in the eastern part of the watershed covered mainly by perennial C 4 grasses. The elevation is 1526 masl. Soils consist mainly of very gravelly sandy loams which contain limestone rock fragments. Canopy height is estimated 0.4 -0.7 m. Slopes are 4-9%. Average temperature is 19.3 °C.