Impact and Management of Small Farm Dams in Hawke's Bay, New Zealand

<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>

Impact and Management of Small Farm Dams in Hawke's Bay, New Zealand

<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>

The impacts of peat slides on upland blanket peatland hydrology, ecology and soil structure. A paired catchment approach.

&lt;p&gt;Rainfall-induced landslides are difficult to forecast and often evolve into highly destructive flows, as such, they are one of the most dangerous natural hazards globally. While our understanding of peatland hydrology has improved greatly in the past two decades, there has been less focus on the response of peat hydrology following perturbations such as wildfires and landslides. &amp;#160;Here we report on a new paired catchment experiment in Ireland. Our focus is to quantify the hydrological changes following peat landslides and further, to establish the short-term and longer-term impacts on local peatland hydrology, ecology and recovery.&lt;/p&gt;&lt;p&gt;The two paired sites are located in Co. Leitrim, Ireland, in two adjacent, small upland blanket bog catchments. The first peat catchment (0.2km&lt;sup&gt;2&lt;/sup&gt;) is an area of a recent (June 2020) slope failure. According to preliminary estimates ~178,000 &amp;#8211; 188,000 tonnes of peat were transported downstream during the peat slide event, resulting in a large landslide scar section (~0.059 km&lt;sup&gt;2&lt;/sup&gt;) in a special area of conservation [SAC]. Preliminary impacts are assessed to include: habitat loss, decreased slope stability, impacts on hydrology and water quality, as well as increased local erosion.&lt;/p&gt;&lt;p&gt;This catchment is paired with an adjacent upland blanket peat catchment (0.11 km&lt;sup&gt;2&lt;/sup&gt;) which is deemed to have been under the same anthropogenic pressures (grazing, upslope forestry plantation).&lt;/p&gt;&lt;p&gt;A hydrometric suite, including weather station, piezometers, and water level recorders to evaluate the surface and subsurface hydrology has been installed at both sites. In addition, we are monitoring the response of landslide deposits (e.g. rafted peat, some with still-standing sika spruce), ecology, soil structure, permeability and shear strength in both catchments.&lt;/p&gt;&lt;p&gt;Here we will report on the initial results of our monitoring.&lt;/p&gt;

Drought-induced non-stationarity in the rainfall-runoff relationship invalidates the role of control catchment at the Red Hill paired-catchment experimental site

The most widely used approaches for estimating impacts of vegetation changes on runoff are the paired-catchment method, the time-trend analysis method, and the sensitivity-based method. These three methods have yielded consistent results in many paired-catchment studies, except at the Red Hill experimental site in Australia. However, reasons for the inconsistency have not yet been identified. The objective of this study was to identify the reasons for the inconsistency amongst results using observations of two paired catchments from 1990 to 2015. Results from these three methods showed that afforestation accounted for 32.8 %, 93.5 %, and 76.1 % of total runoff changes, respectively. The inconsistency in results were still apparent even the longest available observation record was used. The rainfall-runoff relationship of the control catchment has been used only in the paired-catchment method. This relationship was confirmed to become non-stationary during the pre- and post-calibration periods due to a 10-year prolonged drought, leading to the inconsistency amongst results. By eliminating drought's effects on the rainfall-runoff relationship of the control catchment, afforestation’s contribution to runoff reduction was 73.4 % using the paired-catchment method, agreeing well the other two methods. This study not only revealed the reason for the inconsistent results that had long been observed at the famous experimental site, but also proved, using experimental observations, that prolonged drought can induce non-stationary rainfall-runoff relationship in catchment. It also demonstrated that the stationarity test is vital for correct use of historical time series and effective research on ecological hydrology in the case of frequent extreme climate.

Effects of Forest Harvesting on Warm-Season Low Flows in the Pacific Northwest: A Review

Paired-catchment studies conducted on small (< 10 km2) rain-dominated catchments revealed that forest harvesting resulted in a period of increased warm-season low flows ranging from less than five years to more than two decades, consistent with the results of stand-level studies and process considerations. Of the five paired-catchment studies in snow-dominated regions, none revealed a statistically significant change in warm-season low flows in the first decade following harvest, although two exhibited non-significant higher flows in August and September and one had lower flows. Two studies, one of rain-dominated catchments and one of snow-dominated catchments, found that summer low flows became more severe (i.e., lower) about two decades or so following harvest. These longer-term results indicate that indices such as equivalent clearcut area, as currently calculated using monotonic recovery curves, may not accurately reflect the nature of post-harvest changes in low flows. Studies focussed on medium to large catchments (tens to thousands of km2 in area) found either no statistically significant relations between warm-season low flows and forest disturbance, or inconsistent responses. Attempts to synthesize existing studies are hampered by the lack of a common low-flow metric among studies, as well as detailed information on post-harvest vegetation changes. Further fieldresearch and process-based modelling is required to help elucidate the underlying processes leading to the results from these paired-catchment studies and to enhance the ability to predict streamflow responses to forest harvesting, especially in the context of a changing climate. KEYWORDS: streamflow; forestry; low flows; fish habitat; hydrologic recovery

Using paired catchments to quantify the human influence on hydrological droughts

Quantifying the influence of human activities, such as reservoir building, water abstraction, and land use change, on hydrology is crucial for sustainable future water management, especially during drought. Model-based methods are very time-consuming to set up and require a good understanding of human processes and time series of water abstraction, land use change, and water infrastructure and management, which often are not available. Therefore, observation-based methods are being developed that give an indication of the direction and magnitude of the human influence on hydrological drought based on limited data. We suggest adding to those methods a “paired-catchment” approach, based on the classic hydrology approach that was developed in the 1920s for assessing the impact of land cover treatment on water quantity and quality. When applying the paired-catchment approach to long-term pre-existing human influences trying to detect an influence on extreme events such as droughts, a good catchment selection is crucial. The disturbed catchment needs to be paired with a catchment that is similar in all aspects except for the human activity under study, in that way isolating the effect of that specific activity. In this paper, we present a framework for selecting suitable paired catchments for the study of the human influence on hydrological drought. Essential elements in this framework are the availability of qualitative information on the human activity under study (type, timing, and magnitude), and the similarity of climate, geology, and other human influences between the catchments. We show the application of the framework on two contrasting case studies, one impacted by groundwater abstraction and one with a water transfer from another region. Applying the paired-catchment approach showed how the groundwater abstraction aggravated streamflow drought by more than 200 % for some metrics (total drought duration and total drought deficit) and the water transfer alleviated droughts with 25 % to 80 %, dependent on the metric. Benefits of the paired-catchment approach are that climate variability between pre- and post-disturbance periods does not have to be considered as the same time periods are used for analysis, and that it avoids assumptions considered when partly or fully relying on simulation modelling. Limitations of the approach are that finding a suitable catchment pair can be very challenging, often no pre-disturbance records are available to establish the natural difference between the catchments, and long time series of hydrological data are needed to robustly detect the effect of the human activities on hydrological drought. We suggest that the approach can be used for a first estimate of the human influence on hydrological drought, to steer campaigns to collect more data, and to complement and improve other existing methods (e.g. model-based or large-sample approaches).