Riparian Afforestation Effects on Water Yields and Water Quality in Pasture Catchments

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Abstract
The flow records for two pasture headwater catchments for 9 yr before, and 9 yr after riparian afforestation in one catchment were compared. Average rainfall was 1021 mm yr−1. Riparian afforestation reduced water yields by 68 to 104 mm (21–55%) when the Pinus radiata stand was 8 to 10 yr old. Delayed runoff declined by 52 to 93 mm yr−1 (27–63%). Afforestation reduced the quickflow yield in 1 yr (22 mm or 40%). Peak flows declined in small events, were not affected in medium‐sized events, and may have increased in large events. The large reductions in yield indicate that the riparian zone had a disproportionately important influence on catchment hydrology. They are attributed to high transpiration losses from the riparian pine in seasons with water deficits, and higher than usual forest interception losses because of the small‐scale planting. Streamwater sediment, total and dissolved N and P concentrations in these two catchments and another riparian afforested catchment were monitored for 2 yr. Concentrations were generally lower in the completely pastured catchment. Estimated annual sediment, total P, Kjeldahl N, and nitrate exports from the pasture catchment were 31 to 60%, 70%, 61 to 64% and 58 to 74% of those from the riparian afforested catchments in spite of a higher water yield. Possible explanations for the poor water quality in riparian afforested catchments are described including the lack of riparian wetlands, in‐stream vegetation, and close riparian ground cover. The consequences of riparian afforestation in pasture catchments may not readily be predicted from the impacts of complete catchment afforestation.

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