Rainfall interception by bracken in open habitats — Relations between leaf area, canopy storage and drainage rate

A rainfall simulator has been used to investigate how the free throughfall coefficient p, canopy storage C, and drainage rate Ds, of bracken varies with projected leaf area index (LAI) over a LAI range of 0.4 to 5.88. For field canopies p was found to be a simple exponential function of LAI. Measured maximum water shortage, Cmax was related to LAI by Cmax = 0.467 (± 0.004)LAI. Attempts to relate the measured storage and drainage rate using the modified Rutter drainage function and the recently proposed Calder drainage model were unsuccessful, primarily because both assume zero drainage at zero C. The experimental data show that C always has some positive value Cmin when drainage from the canopy ceases. Cmin was related to LAI by Cmin = (0.156 ± 0.004)LAI, and is smaller than reported values∗ Two new asymptotic drainage functions were fitted to the data, using optimisation with excellent results: Ds = [eK(C−Cmin)] − 1 (I) Ds = A(C − Cmin)K (II) optimisation of eqns. (I) and (II) for each experiment showed that the values Cmin, K and A were simple functions of LAI. Thus both empirical expressions could be expressed as simple functions of LAI and storage, and hence generalized over the complete LAI range. Model (I) explained over 90% of the variance of Ds over the LAI range 0.4 to 5.88. If remote sensing techniques are used to obtain values of LAI, the functions presented have wide applicability to bracken growing in open habitats.