Water cycling in a Bornean tropical rain forest under current and projected precipitation scenarios

Southeastern Asian tropical rain forests are among the most important biomes in terms of annual productivity and water cycling. How their hydrologic budgets are altered by projected shifts in precipitation is examined using a combination of field measurements, global climate model (GCM) simulation output, and a simplified hydrologic model. The simplified hydrologic model is developed with its primary forcing term being rainfall statistics. A main novelty in this analysis is that the effects of increased (or decreased) precipitation on increased (or decreased) cloud cover and hence evapotranspiration is explicitly considered. The model is validated against field measurements conducted in a tropical rain forest in Sarawak, Malaysia. It is demonstrated that the model reproduces the probability density function of soil moisture content (s), transpiration (T-r), interception (I-c), and leakage loss (Q). On the basis of this model and projected shifts in precipitation statistics by GCM the probability distribution of Ic, Q and, to a lesser extent, s varied appreciably at seasonal timescales. The probability distribution of Tr was least impacted by projected shifts in precipitation.