Measuring and modelling the rainfall interception loss by hedgerows in southern England

Gross rainfall (PG), net rainfall and stemflow were measured for nearly 3 years in two hedgerows in southern England. The width of the zone where net rainfall was affected by the hedgerows was equivalent to about two hedgerow heights. Rainfall interception was calculated as the difference between the volume of water, from gross rainfall, that would have reached the ground of the sampling area (which also included areas outside the canopy) without the presence of a hedgerow and the actual amount of net rainfall plus stemflow. Averaged over both hedgerows the interception loss during the period of full leaf cover was 57% of PG if related to the ground area covered by the hedgerow canopies or 24% of PG if related to the total ground area affected by the presence of the hedgerows. For the leafless period, the respective values were 49 and 19%. Stemflow constituted a small part in the water balance of the hedgerows and equalled 0.2% of PG in the summer and 0.5% of PG in the winter. Interception storage capacity, if related to projected canopy area, was 2.6 mm during the growing season and 1.2 mm in the leafless hedgerows. During many small rainstorms, which were often associated with high windspeeds, the hedgerows intercepted more rainwater than the amount that would have fallen on the ground covered by them without their presence. This caused the coefficient of free throughfall, when calculated per unit projected canopy area, to be negative. The original Gash [Gash, J.H.C., 1979. An analytical model of rainfall interception by forests. Quart. J. R. Meteorol. Soc. 105, 43–55] analytical model of rainfall interception was parameterised for the hedgerows and tested using data that had not been included in the parameterisation process. The Gash model predicted the interception loss of hedgerows from daily rainfall data with reasonable accuracy.