A. Granier

A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands

This paper presents a daily water balance model where the main aim is to quantify drought intensity and duration in forest stands. This model requires daily potential evapotranspiration and rainfall as input climatic data. Required site and stand parameters are only maximum extractable soil water and leaf area index, the latter controlling (i) stand transpiration; (ii) forest floor evapotranspiration; and (iii) rainfall interception. Other informations, like root distribution and soil porosity, can be used if available, improving the simulation of short term soil water recharge. Water stress is assumed to occur when relative extractable soil water (REW) drops below a threshold of 0.4 under which transpiration is gradually reduced due to stomatal closure. The model was calibrated using sap flow measurements of stand transpiration in oak and spruce stands during several successive dehydration–rehydration cycles. Validation of the model was performed by comparing predicted soil water content to weekly neutron probe measurements in various forest stands and climatic conditions. The model simulated accurately the dynamics of soil water depletion and recharge, and predicted the main components of forest water balance. Day-to-day estimates of soil water content during the growing season allows to quantify duration and intensity of drought events, and to compute stress indexes. A dendroecological application is presented: a retrospective analysis of the effects of drought on radial tree growth, based on long term climatic time series, is shown. Some limitations and potential applications of the model are discussed.

Evaluation of Spruce Stand Transpiration By Sap Flow Measurements in the Vosges Mountains (Strengbach Catchment-France).

The Strengbach catchment is a multidisciplinary site for hydrological, geochemical and forest research in France. In order to record internal water transfers accurately, water balance measurements have been carried out at the scale of forested stands.