Hydrologic response and runoff model parameters in the New Zealand coastal zone: a study of two urbanising catchments

Abstract

Determining hydrologic response in coastal catchments is problematic, and there is little guidance available for estimating hydrologic model parameters. This study provides measured data from two rapidly urbanising New Zealand coastal catchments, the Mazengarb Stream catchment (Paraparaumu) and a small catchment in Papamoa (Tauranga), to: determine the effect of urbanisation on hydrologic response, derive hydrologic model parameters and explain parameter variability, and investigate the potential for regional runoff parameter prediction for New Zealand coastal catchments. Water levels were monitored for 8 to 12 months to generate flow data from 4 subcatchments within each of the Mazengarb and Papamoa catchments. The subcatchments were small (less than 3.5 km2) and ranged in land use from rural to moderate-density residential. The hydrologic modelling techniques selected (and their corresponding runoff parameters) were the Rational Method (runoff coefficient), the SCS loss technique (curve number) and the Clark synthetic unit hydrograph method (storage factor and time of concentration). The results showed a strong correlation between land use and hydrologic response. The change from rural to residential land use resulted in: baseflow becoming negligible, a significant decrease in the time of rise, peak flow increasing by 300-900%, the runoff coefficient increasing from 0.05-0.08 to 0.18-0.23 and the curve number increasing from around 55-59 to 76-88. Although considerable intra-subcatchment variation in the runoff coefficient and the curve number was observed, some of this variation could be explained by event characteristics. Inter-subcatchment comparison showed that the runoff coefficient and the curve number of the subcatchments of the same land use had statistically similar values. The values were significantly higher for the moderate-density subcatchments and the inter-subcatchment variation was best explained by the percentage of impervious cover. Less inter-subcatchment variation in the Clark storage factor was observed, and was best explained by channel length and subcatchment area. Some verification modelling using HEC-1, the derived Clark storage factor, empirically-estimated times of concentration, and the subcatchment median curve numbers produced satisfactory results. This suggests that the parameter estimates can be used with considerable confidence in the study areas. This modelling highlighted that small catchments need to be included in model calibration, and further quantification of the temporal variability of the curve number is required for accurate modelling. The results highlighted that a range of event magnitudes and careful catchment selection is vital for accurate regional calibration. Further investigation into accurately defining the time of concentration is important for developing regional relationships for the Rational Method runoff coefficient. Nonetheless because the parameters can be related to catchment characteristics and because changes in location did not affect the results the development of regional relationships to predict hydrologic modelling parameters in coastal catchments appears viable. The modelling results suggest that the selected models may be easily manipulated to account for inter-catchment differences in size or the percentage of impervious cover within the coastal zone. As such, the results presented provide a basis for predicting the effect of urbanisation on high-frequency hydrologic response in small coastal catchments.