Impact of Deforestation on Clouds and Rainfall On the Northern Part of the Proposed Mesoamerican Biological Corridor

Central America exhibits the typical pattern of complex deforestation now seen throughout the tropics. The region is a mixture of lowlands, mostly converted to agriculture, and mountainous regions, where pristine forests still persist. To protect the biodiversity of this region from further loss, a network of biological corridors and protected areas has been proposed by the governments of Central American countries and international organizations. The present study examines the impact of deforestation in the northern part of Central America on the proposed corridor network, the Mesoamerican Biological Corridor. We use high-resolution numerical model simulations using the Colorado State University Regional Atmospheric Modeling System (CSU RAMS) to study the impact of three types of conditions: 1) pristine, 2) current and 3) extensive deforestation. In addition, GOES-8 satellite imagery is used for comparing with the numerical simulations of cloud formation. Since vegetation in the proposed protected areas would is under maximum stress in the dry season, this study is focused in the dry season month of March. During the dry season, the soil dries progressively from the soil surface down to increasing depths. Contrary to expectations, in-situ measurements of soil moisture in Costa Rica show similar values both in forests and pastures in the dry season. Measured soil moisture values in March are around 10% of the field capacity in the upper few centimeters, increasing to values of around 30% at depths of 1 m. Yet, observations show that the vegetation in pasture regions is stressed at this time while vegetation in the forested regions is not affected, implying that the forest vegetation is accessing deep soil water. Similar behavior is expected in other regions of Central America. This observation has significant implications to the design of the numerical modeling experiments. Currently the vegetation parameterization used in the RAMS does not specify rooting depth greater that 2m for any ecosystem type. To properly characterize the surface energy budget, the root water uptake was modified based on our field observations to allow for extraction of water from deeper soil layers by the trees. We also used the more recent ecosystem database generated at the University of Maryland from the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery to simulate current conditions. Leaf Area Index (LAI) derived from MODIS satellite observations are used in the simulations and values prescribed for forested and deforested conditions. In addition a soil database was added to RAMS based on the one-degree Food and Agricultural Organization (FAO) soil types and depths. These additions add considerable new capabilities to accurately model conditions in Central America. These changes result in significant modification to the surface energy budgets together with the modification of the thermodynamic profile of the near surface atmosphere during the dry season. We demonstrate that deforestation has serious consequences for the proposed biological corridors. Surface air over deforested areas tends to get warmer and drier, and when these winds flow over forested patches they impact the cloud formation processes and rainfall. Numerical simulations show the locations where these impacts will be the largest, potentially making these regions climatically unstable. The managers of the Mesoamerican Biological Corridor project can utilize this information for better planning.