A distributed model of water balance in the Motueka catchment, New Zealand

A distributed water balance model is used to simulate the soil moisture regime of the Motueka catchment. The model is a major simplification of the Distributed Hydrology–Vegetation–Soil Model (DHVSM) with modifications suitable for the study area. The model was applied at 25-m resolution with a 1-day time-step for 10 years. The simulated hydrograph showed good correspondence with the observed hydrograph and there was good agreement of simulated and measured mean annual discharges (57.3 m3 s−1 as compared with 58.7 m3 s−1). Five different land cover scenarios were used to predict the effects of vegetation change on the hydrological regime: (1) current land cover; (2) prehistoric land cover; (3) maximum pine planting; (4) pine trees on easy slopes; and (5) pine trees on steep slopes. The pine scenarios all reduced the mean annual flow by about 2 m3 s−1, while the prehistoric scenario reduced the mean annual flow by about 6 m3 s−1. The pine scenarios (3, 4, and 5) reduced the 7-day 5-year low flow from 7.4 m3 s−1 to between 6.5 m3 s−1 and 6.8 m3 s−1, respectively; and the prehistoric scenario reduced the 7-day 5-year low flow to 5.3 m3 s−1.