Assessing Water Availability and Unmet Water Demand Using the WEAP Model in the Semi-Arid Bweengwa, Kasaka and Magoye Sub-Catchments of Southern Zambia

Located in the semi-arid zone of Zambia, the Mutama-Bweengwa, Kasaka and Magoye sub-catchments have witnessed a high demand for water due to increase in population and socio-economic activities putting more pressure on water resources. This study assesses the hydrological components and ascertains the available water resources and unmet demand in the sub-catchments using the Water Evaluation And Planning (WEAP) Model and hydrometeorological data collected between 1951 and 2018. The model was calibrated and validated on 1971-1981 and 2008-2018 data respectively. The results reveal that the sub-catchments have transitioned from positive to negative water balance with −164.295 Mm/year for Mutama-Bweengwa, −19.021 Mm/year for Kasaka and −86.368 Mm/year for Magoye. Evaporation was 1815.259 Mm/year for Mutama-Bweengwa, 1162.655 Mm/year for Kasaka and 1505.664 Mm/year for Magoye. The demand for water has been increasing over time for various purposes such as irrigation, domestic, urban/rural water supply and livestock. The overall water storage in the sub-catchments showed a negative water balance for the year 2018. The observed and simulated peak streamflow were 8.16 m/s and 7.7 m/s occurring during the month of January and February respectively. The WEAP model performance achieved R of 0.98 during calibration and 0.95 for validation, and an NSE of 0.83 for calibration and 0.85 during validation. The values of objective functions show that the hydrology of the Mutama-Bweengwa, Kasaka and Magoye sub-catchments as predicted by the WEAP model provides satisfactory confidence for prediction of future streamflow and hence projection based on future scenarios. How to cite this paper: Tena, T.M., Nguvulu, A., Mwelwa, D. and Mwaanga, P. (2021) Assessing Water Availability and Unmet Water Demand Using the WEAP Model in the Semi-Arid Bweengwa, Kasaka and Magoye Sub-Catchments of Southern Zambia. Journal of Environmental Protection, 12, 280-295. https://doi.org/10.4236/jep.2021.124018 Received: March 11, 2021 Accepted: April 26, 2021 Published: April 29, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Background
Water is an important resource that drives economies and social well-being of human societies. Understanding the processes that control the existence of water resource, its variability in time and space, and the ability to quantify its availability is important for its sustainable management and efficient allocation among competing users [1]. Water availability and scarcity are a concern not only for future but they are now a reality in the developed world [2] as well as the developing world [3]. Non-availability of water has the potential to retard desirable economic and social development in a given society. Conflicts over future water allocations for various purposes have been reported both in advanced countries [2] and the developing countries. Therefore analysis and quantification of water availability, water use and water management are key issues. Water availability is simply the supply of water in excess of that currently allocated for consumptive use in a particular basin; that is, the amount of water available for new development. It is one of the most important indicators of sustainable development in modern society. According to Tidwell [4], population growth coupled with industrial and agricultural expansion brings about new demands for water, further putting more stress on water availability. As observed by Barlow [5], water availability and use greatly depends on the basin hydrology, climate, use characteristics, legal and regulatory institutions, and personal values of the basin inhabitants. Characterizing water availability requires data on both water availability and the dynamics of water resources use in a region.
The question that arises then is "where in the catchment and how might the development of new agricultural, settlement and industrial projects be challenged by limited water availability?" Answering this question requires data characterizing the current water use in a catchment and the hydrological processes such as precipitation, interception, evapotranspiration, infiltration, surface runoff, groundwater recharge, sediment yield, streamflow and catchment storage. Streamflow strongly influences the hydrology of a basin. For instance, streamflow reduction affects water supply for human consumption as well as maintenance of in streamflow requirements for other environmental needs [5]. Streamflow is one of the most important components in hydrological processes and exhibits spatial and temporal variability. It plays an important role in water resources assessment and management. Understanding the characteristics of the streamflow is vital in the hydrological modelling of a catchment [6]. Long-term analysis of streamflow  [11], to explore a wide range of demand and supply options for balancing environment and development [12], to determine relations between reservoir storage, in streamflow, and water supply yield [13], to investigate and assess scenarios of future water resource development under different demands [14] [15] [16] and under irrigation and climate change scenarios [17], and to model the response of small multi-purpose reservoirs to hydrology for improved rural livelihoods [18]. It is for this reason that the WEAP model was adopted and applied in this study. The study then recommends sustainable catchment protection and river restoration strategies based on the findings.

Study Area
The Mutama-Bweengwa, Kasaka and Magoye water users are located across   [19]. The elevation within the sub-catchments ranges from approximately 1225 m at the headwaters to 1000 m above sea level at the confluence with the Kafue River in the Kafue Flats [7]. The mean annual rainfall and temperature range between 650 mm to 800 mm and between 12˚C -26˚C respectively (WARMA, 2018). The three sub-catchments are located in Zambia's Agro-ecological Zone IIa [20] which is warm semi-arid conditions and annual climatic variation as indicated in the Compendium of Environmental Statistics [21].
In terms of livelihood, local communities inhabiting the three sub-catchments are largely subsistence farmers depending on rain-fed agriculture. However, the headwater streams of the sub-catchments support a number of commercial farmers. Land use activities such as industrial, brick-molding, sand mining, logging and charcoal production exist in the sub-catchments mainly at a small scale [22]. As shown in Figure 2, subsistence crop farm land is the dominant land use in the three sub-catchments and crop land cover has increased in the last decades, which contributed to loss of natural forests. The flat landscape parts of the sub-catchments are sparsely wooded grassland and prone to inundation due to increased runoff. The common types of vegetation in the sub-catchments include floodplain grassland, termitaria grasslands and woodlands.

WEAP Model Data Inputs
The main types of inputs considered during the configuration of the WEAP  model were climate data and physical data. The climate data used were rainfall, temperature, relative humidity, wind speed and solar radiation. These data inputs where obtained from the Zambian Meteorological Department. Physical data inputs used were land use/land cover ( Figure 2), soil information and catchment water uses. As {shown in Table 1 in which the priority orders 1 to 4 indicate that WEAP will first allocate all water demands to priority 1 node untilall water needs are met, then priority 2 water demand will follow in that order. Observed streamflow data were also used during hydrological calibration of the model. Catchment water use priorities are ranked based on the Zambian Water Management Act 2011 and Kafue Catchment water uses and allocation criteria proposed by the Water Resources Management Authority [23]. The theoretical aquifer storage capacity in Mutama-Bweengwa, Magoye and Kasaka sub-catchments were 11,176.69 Mm 3 , 3704.28 Mm 3 and 3357.19 Mm 3 respectively. These were estimated from WARMA's groundwater database and the BGR/GReSP Southern Province groundwater report [7] and were used respectively as input in the WEAP model. Two dams in every sub-catchment were selected to represent hydraulic structures. These were used to estimate storage capacity Q [Mm 3 ] using Equation (1) Figure 3 shows the configuration and calibration of the WEAP model based on

Model Configuration and Calibration
of the observed discharge, and sim Q is the mean of the simulated discharge. The model performance was further evaluated using the PBIAS.

Available Water in the Sub-Catchments
The estimation of annual available water balance for selected years in the sub-catchments in Mutama-Bweengwa, Kasaka and Magoye was computed using the general water balance equation shown in Equation (3). All the main water balance components namely, Inflows, Outflows and Change in Storage (ΔS) were taken into account. Total water inflows into the sub-catchment must equal total water outflows from the sub-catchments [10].

P ExtIn ET Outflow ABST S
where P is precipitation, ExtIn is the amount of flow into the catchment besides precipitation, ET is evapotranspiration, Outflow is outflow from the catchment, ABST is the total abstraction for various uses and ∆S is the change in catchment storage.

Model Performance
The performance of the model assessed for goodness-of-fit statistically achieved a Nash-Sutcliffe efficiency (NSE) of 0.83 (83%) for calibration and 0.85 (85%)  [25]. The purpose for calibrating and validating the model before the scenario implementation is to achieve the naturalized streamflow to be used during water demand and allocation modelling. Based on the calibration and validation results, the performance of the WEAP hydrological model was considered satisfactory and therefore fit to achieve optimal analysis of water demand and allocation [26] in the Mutama-Bweengwa, Kasaka and Magoye sub-catchments and assess the water demand and allocation under different future scenarios [10].

Water Balance (Available Water) for Selected Years
The water balances of the Mutama-Bweengwa, Kasaka and Magoye sub-catchments for selected years are as summarized in Table 4. The hydrological balance components were assessed and a hydrological model has been established for the respective sub-catchments. As shown in Table 4 The overall storage water of the respective sub-catchments in the year 2018 indicated a substantial reduction as shown in Table 5. Generally, the water balance estimates indicated that there is a negative water balance in the sub-catchments for the year 2018.    10% reduction in unmet water demand for this scenario. Table 5 gives scenario comparison between current-day and improved agriculture with e-flow requirement implementation showing a reduction in the unmet demand in the sub-catchments. The reduction in unmet demand from 113 Mm 3 to 88 Mm 3 for the Scenario 2a (Current-day Scenario) implies 78% of the assured met water demand in the sub-catchments. Improved agricultural practice with e-flow and 30% increase in rainfall scenarios showed a significant reduction in unmet water demands in the Sub-catchment between Scenarios 2a and 3a. The reduction in unmet water demand from 140 Mm 3 (under scenario 3a.) to 115 Mm 3 (under scenario 2a.) was observed in the month of October while the minimum unmet water demand of 40 Mm 3 for both scenarios was indicated in the month of February ( Figure 6).

Unmet Water Demand under Different Scenarios
The increased commercial agricultural scenario showed an increase in unmet water demand increase from 140 Mm 3 to 207 Mm 3 particularly in the month of October representing a 67% rise. This is likely to further stress the water system in the sub-catchments. The minimum unmet water demand under increase commercial agriculture was noticed during the month of February as 115 Mm 3 .
The scenario assessment shows, different way to river restoration and allocation of available water to the competing users in the sub-catchments. Changing cropping patterns (scenario 2a) while maintaining the size of the agricultural area shows reduction of stress on water resources and hence improving the availability of the resources to the users as compared to the results obtained under current day demands. Comparing to the current day (reference/baseline scenario), implementation of scenario 2a and 3a showed achievable results towards  river restoration and improved water availability in the sub-catchments while scenario 3b and the increased commercial agriculture scenario impacted negatively in the sub-catchments through the noticeable increased unmet demands. The variation in monthly water demand at the 13 demand sites given in Table   5 is further illustrated in Figure 7.

Conclusions
The study has determined the available water resources in the Mutama-Bweengwa, Kasaka and Magoye sub-catchments and the water demand under different future scenarios using historical hydrological data in the WEAP model. It has revealed that there is an increase in the water demands for various purposes, with commercial agriculture becoming the largest water consumer in the sub-catchments.
1) The three sub-catchments are clearly experiencing an increase in water deficit. If not addressed, the water scarcity may trigger conflicts over future water allocations amongst the various water users and could retard the desired socio-economic in the sub-catchments.
2) The total unmet water demand for all types of uses in the sub-catchments 3) The total unmet demand for future scenarios in the sub-catchments for all types of uses is 88 Mm 3 under improved agriculture and e-flow implementation, 103 Mm 3 under 30% increase in rainfall and 128 Mm 3 under 30% reduction in rainfall.
4) In all scenario evaluations, the maximum unmet water demands were felt during the month of October while the month of February had the minimum unmet water demand. 5) Improved agricultural practices (while maintaining the size of cropland) with e-flow requirement implementation comparatively reduced the water demand in the sub-catchments thereby improving the coverage and unmet water demands.
6) Meeting the water demand of the ever-growing population and associated socio-economic development activities in the sub-catchments is possible. However, this requires appropriate water resources management and monitoring interventions.

Recommendations
Based on the findings and conclusions of this study, the following recommendations have been proposed: 1) Implementation of improved agriculture practices and e-flow (scenario 2a) to restore the Magoye River system, improve the catchment water availability and reduce the unmet water demands in the sub-catchments.
2) Implement water harvesting techniques such as small reservoir construction, ponds, check dams and afforestation activities to capture peak river runoff for usage in the dry period (The hydrological flow pattern in the sub-catchments is flashy).
3) Implementation of integrated catchment strategies through formation of water user associations [10]. 4) Groundwater exploration through the construction of communal kiosks to improve reduction in the unmet catchment water demands. 5) Development and implementation of a water user monitoring and information platform similar to Kaleya Catchment portal [27]. 6) Establishment of hydrometric stations in the respective sub-catchments to improve water resources monitoring and water availability estimations for various user allocation.