Comparison of land vs floating evaporation pans to measure evaporation from dairy farm effluent ponds in the Waikato Region, New Zealand

The Waikato region contains the most dairy farms in New Zealand at approximately 4,520 (Cohen & Hackwel, 2018). With the intensity of dairy farming gradually increasing over the years, a number of negative environmental impacts have been linked to dairy farming (Scarsbrook & Melland, 2015). Recent reports have shown that non-conformance is widespread in the Waikato Region (Cohen & Hackwel, 2018). Dairy effluent ponds are a major source of potential contamination for water resources and can be tested for seepage using a water balance analysis test. A water balance analysis test requires highly accurate evaporation estimation to produce a definitive seepage figure (B. Parker et al., 1999). Existing testing methods utilise a non-standard land-based evaporation pan for their estimations. Previous research has widely concluded that a floating pan can produce evaporation estimates that are closer to the actual value experienced by the water body (Ham & DeSutter, 1999; Rich, 2004; Klink, 2006; Masoner et al., 2008) The aim of this project was to compare a floating evaporation pan with its land based equivalent. A custom floating pan was designed and constructed taking into account consideration of both prior research and its specific operational requirements. The floating pan was installed alongside the existing land evaporation pan and a supporting mobile weather station at four effluent pond test sites under 0.2ha in the Waikato region. Unfortunately, delays prevented additional tests from being undertaken. For each test the water level in the pans were continuously monitored over three days and the data compared using both a numerical summary and plotted graphs. Overall, the data from the floating pan was smoother, more consistent, and more resilient against large fluctuations caused by rapid temperature change. It could also be seen that the land based evaporation pan experienced greater evaporation than the floating evaporation pan which is supported by the existing literature (Rich, 2004). The data also showed that the land pan was subject to exaggeration of rainfall. To confirm the preliminary trends observed in the currently completed testing it is recommended that this research be continued to expand the test data pool. The preliminary results show that a floating evaporation pan will provide the seepage test with more accurate and consistent data that will contribute to a higher accuracy seepage estimate. Accurate seepage testing with simple and effective equipment like the floating evaporation pan will help ensure that effluent dairy effluent ponds are being checked reliably. This will in turn support the mitigation of freshwater contamination as result of seeping effluent ponds