摘要
With increasing patterns of climate change and variability, water resources for agriculture may become more unpredictable. The possibilities of decreased precipitation and increased competition for water resources will be especially important for farmers who depend on rainfed agriculture. A study of coffee agroforestry systems in Southern Mexico (Chiapas, Mexico) was conducted to examine the ability of shade trees to maintain water availability for the coffee crop in a shade agroecosystem. Soil moisture, soil evaporation rates, and the evaporative transpiration potential of coffee plants were measured to examine the amount of water available to coffee plants and potential amount of water lost by the soil and coffee plants in systems under varying levels of shade cover. Soil evaporation and evaporative demand for crop transpiration were compared in coffee systems under different levels of shade canopy during both the wet season and dry season between July 2004 and June 2005. With 60–80 % shade cover, daily soil evaporation rates significantly decreased by 41 % compared to the low shade site (10–30 % shade), although high levels of soil moisture were maintained in the dry season with only 30–65 % shade cover. Coffee transpiration demand was strongly affected by shade cover as shade cover affects microclimate and the radiant energy within the system. Microclimate factors (light, temperature, and air saturation vapor pressure deficit) showed strong correlations to evaporative demand as a result. Shade cover ≥30 % showed significant reductions of 32 % in evaporative transpiration demand when compared to the low shade site. The presence of shade cover in agroforestry systems is capable of reducing overall evaporative demand from soil evaporation and coffee transpiration, therefore offering a higher level of crop protection for farmers with agricultural vulnerability to reduced water resources.