DRIFT-ARID: Application of a method for environmental water requirements (EWRs) in a non-perennial river (Mokolo River) in South Africa

Abstract

Methods developed to determine the amount of water required (EWR) to sustain ecosystem services in non-perennial rivers need a different approach to those used in perennial rivers. Current EWR methods were mostly developed for use in perennial rivers. Non-perennial rivers differ from perennial ones in terms of variability in flow, periods of no-flow and related habitat availability. A DRIFT-ARID method (an adaptation of the Downstream Response to Imposed Flow Transformation (DRIFT) method) was developed, tested and adjusted, using the semi-permanent Mokolo River. Field data from five study sites was collected from April to May 2010 by a multidisciplinary team. The results were used in a DRIFT-ARID Decision Support System (DSS) to determine the impact of five chosen development scenarios in the Mokolo River Catchment. An integrated groundwater-surface water MIKE-SHE hydrological model was used to simulate the hydrology of the chosen scenarios. Specific non-perennial river indicators such as onset of dry phase were identified and included in the DRIFT-ARID DSS. DRIFT-ARID has the potential to be used in non-perennial rivers and, once set up, can provide results for future scenarios. The method now needs to be tested on other non-perennial river types, especially episodic rivers where data are scarce or non-existent.

MIKE-SHE integrated groundwater and surface water model used to simulate scenario hydrology for input to DRIFT-ARID: the Mokolo River case study

“…This is the third in a series of three papers, which should be read in sequence, which present the structure and activities of DRIFT-ARID (Seaman et al, 2016a); a test application on the Mokolo River System (Seaman et al, 2016b) and the integrated groundwater-surface water hydrology component for input into the DRIFT-ARID method (this paper).…”

“…Sites 1, 2, and 3 are situated upstream of the Mokolo Dam and Sites 4 and 5 downstream. For complete site descriptions, see Seaman et al (2013).…”

“…A full list of indicators chosen is available in Seaman et al (2013;2016a,b). Most of the values for each indicator could be derived from a small set of model outputs for each DRIFT-ARID study site:…”

“…With the present condition of the river ecosystem described, simulated flow response to any potential water-related management intervention of interest can then be interpreted in terms of the predicted change in physical, chemical, and biological responses relative to the PD conditions. The final hydrological output of an EWR is a description of flows needed to maintain a range of possible future ecosystem conditions that would be brought about by the different management interventions (King and Pienaar, 2011;Seaman et al, 2010Seaman et al, , 2013.…”

“…One of the main requirements of the DRIFT-ARID method (Seaman et al, 2013;2016a,b) was that it include an integrated or coupled groundwater and surface water hydrological model to produce a daily time series of hydrologic and hydraulic indicators to pass to the DRIFT DSS for each development scenario. After consultation with prominent hydrologists in South Africa and abroad, the research team decided to use the MSHE code (MIKE SHE/MIKE11; Graham and Butts, 2005), developed by DHI (www.dhigroup.com).…”