Water Balance and Evaporation for Two Intermittent Estuarine Lakes

Abstract The management of small intermittently open or closed estuaries (ICOLLs) requires the prediction of lake water levels and the probability of breaching of the entrance barrier in real time, seasonally, and long term. The water balance is a key tool that in turn requires prediction of the open water evaporation. The drivers of the water balance of two ICOLLs on the south-east coast of Australia were studied: The two ICOLLs, Durras Lake and Lake Wollumboola, have very different morphologies: Durras Lake is a drowned stream valley with a largely steep, forested catchment, while Lake Wollumboola is a back-dune lagoon ICOLL with a wider, shallow water body. For these small estuaries, hydrologic and geomorphic data are generally limited or unavailable; hence, methods were developed to use data routinely available, primarily from government agencies, without using regionally averaged or “text book” parameter values. For both lakes, evaporation was shown to be on average larger than either of the inflows from the catchment or from the direct rainfall on the lakes; thus, evaporation must be reliably predicted for use in the water balance. The calculated evaporation agreed with widely used but data-intensive formulas. For each ICOLL a robust linear correlation of lake evaporation with the incident solar radiation accounted for 94% of the variance in the evaporation. The seasonal variation of evaporation fitted a cosine curve, again accounting for 94% of the variance of the evaporation. Validation of the water balances using the evaporation-solar correlation and a derived runoff coefficient provided a close match with the historical record. On most days, the water loss from the lakes by evaporation was larger than the sum of the inflows from catchment runoff and direct rainfall on the lakes. Storms of duration 1–3 days rapidly increased the water storages and the lake levels, potentially leading to breaching of the entrance barrier. This fluctuating balance drives the intermittent behaviour of these ICOLLs and emphasises the importance of accurately assessing the lake evaporation.