Agricultural and Forest Meteorology 100 2000 323–336
Measured and modelled rainfall interception loss from an agroforestry system in Kenya
N.A. Jackson
∗ Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxon OX10 8BB, UK
Received 17 May 1999; received in revised form 6 October 1999; accepted 11 October 1999
Abstract
Rainfall interception losses from an agroforestry system in semi-arid Kenya comprising Grevillea robusta and maize were measured over a period of 33 months. These measurements showed that interception was slightly higher 10.2 under trees
with no maize understorey, than in the intercropped treatment 9.8, and was directly related to the degree of tree canopy cover. Interception estimates using the reformulated version of the Gash analytical model were ∼4 lower than measured
totals, and were strongly dependent on both canopy cover and on monthly variations in the mean rainfall rate ¯ R, mm h
− 1
. ©2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Rainfall interception; Agroforestry; Modelling; Grevillea robusta
1. Introduction
Agroforestry has been promoted as a possible means of increasing the productive use of rainfall
in water-limited environments, by using the water which is usually inaccessible to conventional crop-
ping systems; i.e. soil water reserves that are below the crop rooting zone andor rainfall occurring outside
the normal cropping seasons Jackson et al., 1998. However, any potential increase in rainfall utilisation
by agroforestry systems, when compared to either woodlots or open crop fields, must be offset against
the evaporation of rainfall intercepted by the likely greater canopy size tree and crop, and not therefore
available for crop growth Wallace et al., 1995. In- terception losses must also be weighed against any
increased abstraction of water from the crop rooting
∗
Corresponding author. Tel.: +44-1491-692336; fax: +44-1491-692424.
E-mail address: n.jacksonioh.ac.uk N.A. Jackson.
zone by tree roots, a component of the water balance that can be manipulated through practices such as
root andor canopy pruning. Agroforestry systems are often spatially complex in
nature, with both the tree and crop canopies affecting rainfall distribution and input to the soil surface. Since
quantitative investigations of water-use in agroforestry systems are rare Ong et al., 1991, we decided to in-
vestigate the water-use of a hillslope agroforestry sys- tem in Kenya, combining Grevillea robusta A. Cunn.
ex R. Br. trees with a local variety of maize Zea mays L. cv. Katumani. Water balance components other
than rainfall interception are reported elsewhere; e.g. soil evaporation Jackson and Wallace, 1999a; Wallace
et al., 1999, rainfall infiltration Jackson and Wallace, 1999b, soil water storage Jackson et al., 1999, and
tree and crop transpiration Lott et al., 1997.
It was intended that the experiment would provide data on how rainfall distribution is modified by both
tree and crop canopies in an agroforestry system, as well as demonstrate how these processes could be con-
0168-192300 – see front matter ©2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 4 5 - 8
324 N.A. Jackson Agricultural and Forest Meteorology 100 2000 323–336
trolled by regular manipulation of the tree canopy, achieved through pruning. This paper compares the
measurement and modelling of rainfall interception by G. robusta A. Cunn. ex R. Br. trees in a tropical
agroforestry system in Kenya. The previously refor- mulated version of the Gash analytical model Gash,
1979; Gash et al., 1995 was used to model the inter- ception loss from the discontinuous Grevillea canopy.
Interception estimates from tropical forests are of- ten more uncertain than those obtained in temperate
climates due to the complexity of the tree canopy struc- ture Jackson, 1971; Bruijnzeel and Wiersum, 1987;
Lloyd and Marques, 1988; Asdak et al., 1998a. Rain- fall interception loss in an agroforestry system will
depend on the extent of the tree canopy cover, and this in turn depends on factors such as the tree planting
density at establishment, and subsequent thinning and pruning practices.
Most of the literature comprises studies from closed forest canopies, although some cases exist where in-
terception by discontinuous tree canopies has been studied Rao, 1987; Teklehaimanot and Jarvis, 1991;
Valente et al., 1997; Asdak et al., 1998a. In terms of tropical forestry and agroforestry systems, Veracion
and Lopez 1976 examined interception below Pinus kesiya stands, and interception under multi-storey
systems incorporating coffee and cocoa and shade tree species has been explored by Imbach et al.
1989. Tropical agroforestry systems often comprise fast-growing tree species grown in rotations as short
as 6 or 7 years. Both measurements and models of interception loss are needed to predict the effects of
the tree component on the overall water balance of the system at all stages from establishment to the harvest
of the trees.
2. Materials and methods
