Measuring forest floor and canopy interception in a savannah ecosystem (A case study of Harare, Zimbabwe)

Abstract

Interception is an important process that influences antecedent soil moisture conditions that are important for flood generation. It is however, one of the most underestimated processes of the hydrological cycle. Studies that consider interception focus on canopy interception and neglect forest floor interception. Most investigations on interception have been carried out in Europe and America but little is known about interception measurements in Africa. A study was carried out to measure forest floor and canopy interception in an African savannah ecosystem and to analyse the influence of meteorological factors and vegetation characteristics. Thatching grass (Hyparrhenia filipendula) and Msasa (brachystegia spiciformis) tree leaf litter were used for the study. Two forest floor interception measuring devices were set up. Each device consists of two galvanized steel basins mounted above each other and continuously weighed with strain gauge sensors. Interception from the forest floor was determined by calculating the water balance of the upper and lower basin. Canopy interception was computed as the difference between gross and net precipitation. Sprinkler experiments were carried out to determine the storage capacity of the leaves and grass. Forest floor interception was measured to be 20% of net rainfall for the Msasa leaf litter and 26% of gross rainfall for the Thatching grass. Canopy interception for the study period averaged 25% which is comparable to literature. The maximum water storage capacities for the Msasa leaf litter and Thatching grass were 1.8mm and 1.5mm respectively. The sprinkler experiments showed that water storage capacity increases with intensity until a threshold is reached then it starts decreasing. It is concluded from this study that interception is a threshold process which is affected by meteorological factors and vegetation characteristics. The study also revealed that evaporation ‘loss’ from a litter layer is less than that from grass which is not as tightly packed as the leaf litter layer. However, vegetation with a higher Leaf Area Index (leaf litter) has a higher storage capacity than that with a lower Leaf Area Index (grass). Statistical analysis showed that there is a significant (P<0.05)* relationship between evaporation and canopy interception. Of most importance, the study revealed that combining canopy and forest floor interception yields a value of approximately half the amount of precipitation received thus interception should be given greater consideration in rainfall – runoff studies.