The relationship between throughfall, stemflow, and rainfall in a northern New Zealand native forest headwater catchment

Results are presented for a 14-month multisite throughfall study at Huapai Experimental Catchment, a small native forest headwater catchment near Auckland, New Zealand. Six sites under different vegetation types were instrumented with throughfall troughs and the results empirically related to gross rainfall recorded outside the forest. Relationships between throughfall and gross rainfall, derived for daily data, explain 87-98% of the variance, and are even stronger using non-linear regression on rainfall-event data. Relationships are clearly non-linear in most cases, with evident potential for systematic bias in the derived relationship if a linear model is used for predictive purposes. The throughfall results were combined with an earlier stemflow study at the same location and an additional small stemflow study at one of the six throughfall sites in order to derive non-linear empirical relationships between effective and gross rainfall at each of the six throughfall sites, plus a composite catchment relationship. The key finding is that, although the basic form of the relationship is consistent across vegetation types, there is substantial variability in the magnitude of effective rainfall over distances of a few tens of metres.