Impact and Management of Small Farm Dams in Hawke's Bay, New Zealand
Authors:   , Thompson Jan C. 
Published: November 2021

<p>In New Zealand, thousands of small dams have been built in agricultural areas for the purpose of providing water storage for stock and/or irrigation. These dams interrupt flow on perennial or intermittent streams; however, almost nothing is known of the downstream impact of these dams on flow regime, water quality, sediment transfer, and channel morphology. The cumulative impact of these dams at the catchment scale is likely to be significant. The present research was undertaken in the Ruataniwha Plains of Central Hawke's Bay. With further agricultural intensification in the region, it is expected that the construction of small farm dams will continue as farmers try to secure more on-farm water storage. This study attempts to quantify the effects of these storages in two parts: a paired catchment field study to determine the downstream effects of small dams, and a modelling study to investigate the cumulative impact of these storages on streamflow volumes at the regional scale. Results from the paired catchment field study suggest that the regulation of a small stream by three dams (total storage 11.6 ML) has lowered annual runoff volumes, decreased peak flows, increased periods of low flow, and lengthened the response time of the stream to storm events, as compared to the adjacent unregulated stream. Higher precipitation volumes in the winter act to reduce the degree of these impacts, although flow volumes are still lower as compared to the unregulated stream. Throughout the winter, ponds are full and connected to the downstream system, leading to more days of flow on the regulated stream. The regulation of flow has lowered stream erosion potentials, as evidenced by differences in channel bed sediment and morphological characteristics between the two streams. The regulated channel is aggradational, with no evidence of channel scour found over its length. Water quality changes are also observed, with lower water quality measured in the regulated stream and in the ponds, and generally higher water quality measured in the unregulated stream. The impact of farm dams on streamflow in two regional catchments was investigated using two off-the-shelf models (TEDI, Source Catchments). Model predictions suggest that the current volume of farm dam storage has decreased average annual flow volumes in the two catchments by approximately 1%. The predicted streamflow decrease is more significant under scenarios of future agricultural intensification. Regional climate change scenarios do not show a large effect on catchment streamflow volumes. In comparison to known catchment characteristics, the two models have limitations related to some of the model assumptions, and to the inability of the rainfall-runoff model to accurately represent seasonality of flow in the study catchments. On the whole, the models seem to be biased towards underestimating farm dam impact at the regional scale. The study concludes that farm dams have already influenced catchment streamflow and related processes to some degree. At present, the majority of small farm dams in New Zealand do not require resource consent from local council authorities for construction. It is reasonable to expect that farm dams will continue to be built, and it is important that further construction is undertaken with a sound knowledge of the cumulative impact these dams have on catchment processes and existing streamflow volumes. Proper management will mitigate some of these impacts. Management recommendations include the compilation of an inventory of small dams and their characteristics, continued field investigations, and refinement of a catchment model in order to provide a flexible platform for exploring further management options in the region. This study represents a critical first step towards integrated land and water management in the Ruataniwha Plains and will have relevance for the study and management of farm dams in other areas of New Zealand.</p>