Water storage and evaporation as constituents of rainfall interception

Intercepted rainfall may be evaporated during or after the rain event. Inte rcepted rain is generally determined as the difference between rainfall mea surements outside and inside the forest. Such measurements are often used t o discriminate between water storage and evaporation during rain as well. T wo well-accepted methods underestimate water storage by a factor two as com pared to direct observations. The underestimation of storage is compensated by an overestimation of evaporation during rain by a factor of three. The direct observations of water storage and evaporation appear to agree with p revious direct observations. Thus, it is concluded that these observations are representative Also, our results based on methods using only rainfall m easurements inside and outside the forest appear to agree with previous res ults, This would result in the conclusion that the common methods systemati cally underestimate water storage and overestimate evaporation during rain. Indeed, the systematic errors can be explained by the neglect of drainage before saturation. Water storage is better simulated assuming an exponentia l saturation of a larger storage capacity. A smaller evaporation can be sim ulated using an appropriate resistance to vapour transport. The observation s in dense coniferous forest showed water storage to be the dominant proces s in rainfall interception, but this conclusion should not be generalized t o other forests and climates. Direct observations of water storage and evap oration are recommended to build a realistic set of parameters for rainfall interception studies of the main vegetation types. (C) 1998 Elsevier Scien ce B.V. All rights reserved.