Description

The Okavango Delta and its terminal basins (OD & B) comprise of an inlet upstream in the Okavango River, in Botswana 1st gauged at Mohembo, it flows downstream south-eastwardly in what is known to be the panhandle before spreading out into a series of anastomosing channels that form the swamps. The endorheic system has three terminal basins downstream, Mababe Depression to the east, Lake Ngami more to the south, and the last furthest downstream, Lake Xau located over 200km south-east of the inlet at Mohembo station (Figure 1). Lake Ngami and Mababe Depression have in previous studies been lumped with the main Delta body (swamp) and several hydrological studies and models have been done with the main objective of understanding hydrological dynamics. In contrast, Lake Xau has received less if not none scientific attention. No studies are available addressing the hydrological dynamics of the lake and its key water balance components. Lying in a semi-arid arid environment, evapotranspiration plays a critical role in the water balance of the OD & B with almost 100% of river inflow into the system eventually ending up as evapotranspiration (ET). As a result, it is imperative to improve understanding of the dynamic interaction between surface water, groundwater, and atmospheric demands i.e ET to fully understand the water balance of the Od & B, particularly evaporation at Lake Xau.

Evaporation estimation in Lake Xau is critical as it represents one of the three water balance components, next to river inflow, that can be estimated through the discharge gauging station in Rakops, and lake seepage. The lake evaporation time series will allow improving the understanding of the lake hydrological dynamics. They will be further used in the calibration process of an integrated hydrological model constructed, including the lake in its model domain. The results of the model may improve the understanding of surface-groundwater interactions in the study area.

Objectives and Methodology

Main objective
To estimate the lake open-water evaporation of the intermittent Lake Xau, Botswana

Specific objectives
Define periods when the Lake Xau was filled up with water within the time span from 2010 to 2019.
Define daily evaporation of the Lake Xau over the period from 2010 to 2019 with help of remote sensing and ground monitoring.

Methodology
The RS-based models such as TSEB [Norman et al., 1995], SEBS [Su, 2002], SEBAL [Bastiaanssen et al., 1998], DATTUTDUT [Timmermans et al., 2015], are not directly suitable for estimating ET of open water bodies but they can be adapted to that. The open water evaporation for example can be also defined utilizing surface temperature following methods described in [Wilfired Brutsaert, 1982] and [Wilfried Brutsaert, 2005].

Further reading

Bastiaanssen, W. G. M., M. Menenti, R. A. Feddes, and A. A. M. Holtslag (1998), A remote sensing surface energy balance algorithm for land (SEBAL): 1. Formulation, Journal of Hydrology, 212-213, 198-212.
Brutsaert, W. (1982), Evaporation into the atmosphere, Reidel, Dordrecht, The Netherlands, 299.
Brutsaert, W. (2005), Hydrology - An introduction, Cambridge University Press, The Edinburgh Building, Cambridge, UK, pp 605.
Norman, J. M., W. P. Kustas, and K. S. Humes (1995), Source approach for estimating soil and vegetation energy fluxes in observations of directional radiometric surface temperature, Agricultural and Forest Meteorology, 77, 263-293.
Su, Z. (2002), The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes, Hydrology and Earth System Sciences, 6(1), 85-99.
Timmermans, W. J., W. P. Kustas, and A. Andreu (2015), Utility of an automated thermal-based approach for monitoring evapotranspiration, Acta Geophysica, 63(6), 1571-1608, doi:doi: 10.1515/acgeo-2015-0016.