Comparative nalysis of Hydrologic esponses of Tropical Deciduous and Temperate Deciduous Watershed to Climatic Change’ Jo&

-Lmg-term monitoring of ecological and hydrological processes is critical to understanding ecosystem function and responses to anthropogenic and natural disturbances. Much of the world’s knowledge of ecosystem responses to disturbance comes from long-term studies on gaged watersheds. However, there are relatively few long-term sites due to the large cost and commitment required to establish and maintain them. Knowledge gamed from these sites is also important for predicting responses to future disturbances, such as climatic change, and these sites should be the focal point for the development and validation of predictive models. In this study, we apply a hydrologic model (PROSPER) using climate, vegetation, and soil parameters from watersheds in the mesic southeastern United States and in the dry tropical forests of western Mexico to assess the overall effects of climatic change (increased temperature and [CO& on watershed hydrology. We found that evapotranspiration (ET) increased substantially in both ecosystem types, with increases ranging from 24 to 42%. These increases were directly attributable to changes in leaf energy balance and evaporative demand. Streamflow decreased more substantially, with virtually no streamflow under the greatest temperature increase scenario (+20%) at the site in western Mexico. Decreased stomata1 conductance was not sufficient to offset the effects of increased temperature. Resumen-La hidrologia de 10s ecosistemas forestales arbolados es un integrador clave de respuestas fimcionales al disturbio. El monitoreo a largo plazo de cuencas y &ma proporciona la base para evaluar impactos potenciales de stresses, tales coma el cambio climatico de1 clima, en la fun&n de1 ecosistema. En este estudio, nosotros usamos un modelo enfocado a evaluar 10s hnpactos de1 cambio clim6tico en evapotranspiraci6n en dos ecosistemas contrastantes: coweeta hydrologic laboratory (usa) y la estaci6n de biologfa de chamela (M6xico). coweeta es un sitio de investigacicin ecol6gica a largo plazo (us-her) y es operada por el usda forest serviceresearchychamelaesunaestaci6noperadaporlauniversidad national aut6noma de M6xico. Los dos consisten en una red de cuencas de monitoreo climatico a largo plazo.Coweeta se locahza en las montaftas apalaches de1 oeste de Carolina, usa y se caracteriza por cbma moderado humedo (la temperatura media anual es de 12.6 “C y precipitacibn anual de 1786 mm). chamela se localiza en el e&ado de jalisco en la costa pacffrca de M&ico. La precipitaci6n ‘Paper presented at the North American Science Symposium: Toward a Unified Framework for Inventorying and Monitoring Forest Ecosystem Resources, Guadalajara, Mexico, November l-6,1998. ‘James M. Vose is Research Ecologist, USDA Forest Service, Southern Research Station, locat& at the Coweeta Hydrologic Laboratory, Otto, NC. 3Jose Manuel Maass is Ecologist, UNAM Centro de Ecologia, located at Morelia, Michoacan. media anual es de 707 mm, con patrones estacionales pronunciados (80% de la precipitaci6n cae entre Julio y Octubre), y la temperatura media anual es de 25 “C. Nosotros usamos el modelo hidrolirgico prosper para simular 10s impactos de cambio climatico (aumentb temperatura y ICOzl) en las respuestas hidrol6gicas de estos ecosistemas para evaluar diferencias en la magnitud de la respuesta y para evaluar diferencias en las variables clave. Nosotros tambi6n demostramos la importancia de1 monitoreo a largo plazo para evaluar 10s patrones de respuesta y validar la construcci6n de1 modelo. Watershed scale analyses of hydrologic responses to disturbances requires a commitment to long-term monitoring and an understanding of the basic principles regulating the flow ofwater through the soil-plant-atmosphere continuum. Detecting disturbance caused departures in hydrologic parameters such as streamflow, soil moisture, or evapotranspiration (ET) from “undisturbed” conditions requires knowing the inherent variation in watershed processes, which can only be determined from long-term measurement and analyses. However, establishment and maintenance of longterm hydrologic studies is expensive and hence, there are relatively few gaged watersheds with long-term records in North America. Where small catchment studies have been employed, considerable knowledge on basic hydrologic principles, hydrologic responses to forest management (Swank and Johnson 1994, Swank and Vose 1994), and hydrologic effects of long-term chronic atmospheric deposition (Swank and Vose 1997) has been obtained. Knowledge gained from long-term watershed studies will also be critical for assessing (and detecting) the potential effects of future disturbances, such as climatic change, on watershed processes. Examination of climate-vegetationhydrology responses from historical climatic data provides some insight into potential future responses to climatic change; however, conditions in the next century (i.e., elevated temperature, and [COz]) are likely to exceed the range of variability exhibited in historical data. Hence, modeling hydrologic responses-especially if the model is physiologically based, validated, and parameterized on gaged watersheds-provides a useful tool to evaluate watershed scale responses to climatic change. The hydrologic behavior of a watershed is determined by precipitation input, as modified by soil moisture storage and ET use. Changes in any of these determining factors will influence hydrologic behavior. Climatic change (i.e., increased [COzJ, increased air temperature) has the USDA Forest Service Proceedings RMRS-P-12. 1999 potential to directly alter ET via changes in leaf energy balance, evaporative demand, and stomata1 conductance. If substantial changes in ET occur as a result of climatic change, then watershed hydrologic behavior will also be altered with potential indirect effects on stream water, groundwater recharge, and soil moisture availability. These indirect responses could result into substantial effects on terrestrial and aquatic flora and fauna, and ecosystem processes such as nutrient and carbon cycling. Effects of climatic change on watershed hydrology has generally received less attention than potential effects on productivity. In part, this lack of attention might be due to a general consensus of increased water use efficiency (WUE) under elevated [COz] which is thought to offset drought effects (Norby and O’Neil 1991, Gunderson et al. 1993). However, most of these studies have not quantified the coincident effects of increased temperature on leaf energy balance and the potential feedbacks which might offset WUE gains. Similarly, most of these studies have been conducted under controlled greenhouse or garden conditions, and the soilvegetation-atmospheric continuum has not been adequately represented. The objective of our paper is to use a simulation model, PROSPER, to evaluate the effects of elevated air temperature (with and without changes in WUE) on key hydrologic parameters. Simulations were performed for two contrasting ecosystem types: the dry tropical forests of western Mexico and the mesic temperate deciduous forests of western North Carolina, USA. The primary purpose of this modeling exercise was not to provide exact predictions of watershed response, but rather to demonstrate the usefulness of a combined long-term measurement and modeling approach to understanding watershed processes in two contrasting ecosystems. Comparative studies are particularly useful for determining the robustness of model performance and for determining the general applicability of watershed ecosystem responses. Description of PROSPER We chose PROSPER to simulate hydrologic responses to climatic change because it has been validated and applied successfully in both conifer and hardwood forests in North America (Swift et al, 1975, Huff and Swank 1985, Vose and Swank 1992), and has performedwell in regional ET assessments (USDA 1980). PROSPER was developed from first principles of physiological, physical, and climatological processes regulating watershed hydrology and refined using data from long-term disturbed and undisturbed gaged watersheds at the Coweeta Hydrologic Laboratory (see site description section below) (Swift et al. 1975, Huff and Swank 1985). The combination of physical and biological controls on ET allows the model to be sensitive to changes in air temperature (via effects on energy balance and evaporative demand) and [CO,] (via effects on stomata1 conductance). PROSPER has been described in detail elsewhere (Goldstein and Mankin 1972, Huff and Swank 1985), thus, only a general description is provided here. PROSPER is a phenomenological, one-dimensional model that links the soil, vegetation, and atmosphere. Plant and soil characteristics are combined in an ET surface that is characterized by USDA Forest Service Proceedings RMRS-P-12. 1999 a surface resistance to water vapor loss. This resistance, which is analogous to the relationship between stomata1 resistance and leaf water potential, is a function of the water potential of the ET surface. Evapotranspiration is predicted by a combined energy balance-aerodynamic method that is a function of the surface resistance to vapor loss. PROSPER applies a water balance (through the use of electrical network equations) to the vegetation with the soil divided into layers. Hence, the flow of water within and between soil and plant is a function of soil conductivity, soil water potential, root characteristics for each soil layer, and surface water potential. Soil water flux during unsaturated soil conditions is governed by hydraulic conductivity, where hydraulic conductivity is estimated from the relationship between soil matric potential and moisture content using the procedure described in Luxmoore (1973). The version of PROSPER used in this study simulates ET and soil water redistribution between soil layers on a daily timestep. PROSPER requires the following site-specific climatic data: solar radiation (daily total), precipitation (daily total), windspeed (daytime mean), air temperature (daytime mean), andvapor pressure (daytime mean). With the exception of precipitation, these data are used to compute evaporative demand in the energy balance-aerodynamic equation (Swift et al. 1975). In addition to climatic data, PROSPER requires several key soils andvegetation parameters (Table 1). Parameters shown in Table 1 represent “key” variables which substantially influence the simulation results. For the most part, these parameters were measured directly on the site; however, in some cases “best estimates” from the literature were utilized. To evaluate the effects of climate change and elevated [COs] on hydrologic responses, we examined monthly and annual values of transpiration, ET, and drainage below the lowest soil layer. At Coweeta, drainage has been shown to be reflective of streamflow in all but the driest years (Vose and Swank 1992, Vose and Swank 1994), when drainage from soil water below the root zone is used to recharge a normally saturated deep soil layer. Shallow and coarse soils at Chamela would also suggests that drainage would be reflective of streamflow at Chamela.