Chapter 7 CONTROLS ON PATTERNS OF SOIL MOISTURE IN ARID AND SEMI-ARID SYSTEMS Rodger B. GRAYSON, Andrew W. WESTERN, Jeffrey P. WALKER, Durga D. KANDEL, Justin F. COSTELLOE, and David J. WILSON Department of Civil and Environmental Engineering and CRC for Catchment Hydrology, University of Melbourne, Victoria, Australia - e-mail: rodger@civenv.unimelb.edu.au 1. Introduction Soil moisture is a major control on ecohydrological processes at both the storm event and seasonal scales. It influences the partitioning of precipitation into infiltration and runoff (Chapter 3), is a control on biogeochemical processes (Chapter 11) and is a control on evapotranspiration by limiting water availability to plants (Chapter 3), and so also affecting the partitioning of energy into latent and sensible heat (Chapter 5). In this way soil moisture is a link between the surface energy, water and biogeochemical cycles. In water limited systems such as the arid and semi-arid zones, soil moisture plays a major role in vegetation patterns and type of vegetation cover, and is consequently of primary importance to the ecosystems of these areas (Chapters 1, 15; Hupet and Vanclooster, 2002; Kim and Eltahir, 2004). Figure 1a illustrates a typical one-dimensional conceptualisation of the soil profile and the fluxes that influence the soil moisture stored in the profile. The exchanges between the atmosphere and the soil (precipitation and evapotranspiration) dominate changes in soil moisture (eg. Wilson et al., 2004), with moisture being replenished by infiltration and depleted by soil evaporation and plant transpiration. The relative contribution of evaporation and transpiration depends on the vegetation cover, with transpiration dominating in well vegetated landscapes. Fluxes between the soil profile and groundwater (or deeper parts of the regolith) can be important in some contexts. Drainage from the soil profile is the primary source of recharge for many groundwater systems and capillary rise from shallow groundwater tables can be an important source of water replenishing the profile soil water store during drier periods. In arid and semi- arid environments, interaction with shallow groundwater systems is generally limited to floodplains (Chapter 10) and regionally low areas around lakes. It rarely occurs at the hillslope scale for any significant period of time. Included on Figure 1a is a series of soil moisture profiles measured for a clay-loam soil on a hillslope in Victoria, Australia. Both the amount of soil moisture and its dynamics change with depth. In the upper 50 cm, soil moisture is strongly influenced by the fluxes between the active root zone and the atmosphere, and the moisture here is more variable than the moisture at depth. Surface soil moisture also responds more quickly and so has both short and long time-scale variability, while the moisture at depth is less responsive to short term variations in the fluxes across the soil–atmosphere interface. In arid systems, hillslope soil moisture at depth is typically very low, except following unusually large rain events during which episodic recharge may briefly occur. Figure 1b illustrates a standard conceptualisation of a hillslope. The key difference between Figure 1a and 1b is that lateral flow may now act to redistribute water via both surface and subsurface flow pathways. For significant subsurface lateral drainage to occur the following conditions are necessary: