The Significance of Night Time Evaporation from Irrigation Farm Dams

Summary

Evaporation is a significant loss component from irrigation dams, and although numerous evaporation reduction methodologies exist, their adoption is hindered since they are deemed uneconomical. Any cost/benefit analyses that are done are generally based on poor evaporation estimates due to the complexities involved in evaporation prediction.

One area of uncertainty is the significance of night time evaporation. Similarly, the role of windsheltering on evaporation can be significant but is rarely accounted for in any quantitative assessment. There is therefore a need for improved estimates of evaporation so that water management and investment decisions can have a quantitative basis.

Aims

• To ascertain the importance of night -time evaporation from farm dams across a range of climatic zones.
• To quantify the day -time vs. night -time losses in the different climatic zones and for different seasons.
• To examine and quantify the sensitivity of evaporation predictions to humidity, temperature and atmospheric stability, dam morphometry, wind, wind -sheltering and cloud cover.
• To assess the robustness of the current evaporation prediction framework outlined in the scoping study, and provide an improved framework for engineers to estimate evaporation given routine meteorological and morphological information.

Outcomes

The report shows that the contribution of evaporation during the night is considerable. Predictions from this study using 10 test (modelled) dams in 4 different climatic regions suggest that between 35-45% of the total annual loss of water through evaporation was during the night. i.e. in a climate where potential evaporation is approximately 1.5 m. year-1 the night time fraction equates to roughly 0.6 m.year-1. Where the seepage from dams is measured by assessing the drop in water levels overnight, commonly 10-15% is attributed to night time evaporation. This report shows that the report will be more in the range of 40 - 60 %. It also suggests that night time evaporation is not strongly affected by the dam’s shape.

Background

Evaporation is a significant loss component from irrigation dams, and although numerous evaporation reduction methodologies exist, their adoption is hindered since they are deemed uneconomical. Any cost/benefit analyses that are done are generally based on poor evaporation estimates due to the complexities involved in evaporation prediction. One area of uncertainty is the significance of night time evaporation. Similarly, the role of windsheltering on evaporation can be significant but is rarely accounted for in any quantitative assessment.

There is therefore a need for improved estimates of evaporation so that water management and investment decisions can have a quantitative basis. Specifically, it has been the aim of this project to ascertain the importance of night time evaporation and to provide quantitative estimates of its significance across a range of climatic regions. An additional objective was to compare the current methodology with that presented herein.

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