Understanding climatic-landscape-hydrological interactions at a Meso-scale to guide global change adaptation : a study in the Kaleya river catchment, Zambia

Abstract

This study examined the climatic-landscape-hydrological interactions in a catchment facing landscape fragmentation, agricultural intensification, and increased climatic risks. The study took a holistic approach by examining past, present, and future interactions using the lenses of the green-blue water approach to devise interventions for improved water storage and management in the case study of the Kaleya River Catchment (about 750 km2) of southern Zambia. The results could be extrapolated to other semi-arid areas with similar hydro-geological and climatic settings. To assess the past interactions, a simple landscape hydrology approach was developed and applied to determine factors explaining seasonal water availability and provide insights on how landscape components could be enhanced to augment natural river flows and reduce sediment loss. Based on the Variable Importance in Projection (VIPs), results showed that seasonal climatic and weather extremes involving rainfall intensities, rainfall variability and dry spell length were more important than annual rainfall totals in explaining seasonal water availability. Additionally, patchiness of cover was more important in explaining seasonal water availability than the percentage of cover type in the landscape (PLAND). The Patch Density (PD) and Largest Patch Index (LPI) of reservoirs were the main landscape pattern stressors, alongside percentage of cover type metrics involving PLAND of irrigated cropland and reservoirs. But the LPI of forestland positively explained seasonal river flows. The study recommended that water resource interventions in the region must adapt more to changing seasonal rainfall characteristics than to annual rainfall totals. Additionally, regeneration of larger forest patches could improve river flows. To understand the climatic-landscape-hydrological interactions in the present, naturally occurring stable water isotopes [deuterium (δ 2H) and oxygen-18 (δ 18O)], hydro-chemical parameters [chloride (Cl-1) and electrical conductivity (EC)] were used as tracers. Based on the combination of end member mixing analysis and mixing model analysis, the major streamflow sources could be evaluated. The results revealed that stormwater runoff from non-irrigated areas (43 ± 13) %, the perennial spring (39 ± 21) % and stormwater runoff from irrigated areas (18 ± 17) % were the major streamflow sources in the rainy season. Streamflow sources in the dry season were different upstream and downstream, thereby reflecting different water use dynamics in the catchment. In the upstream catchment, the perennial spring at the river source (65 ± 15) % and irrigation return flows (35 ± 15) % were the dominant streamflow sources. In the downstream part of the catchment, dry season streamflow was mainly attributed to irrigation return flows (73 ± 15) % and wastewater (27 ± 15) %, both associated with water originally transferred in from the adjacent Kafue River through an intra-basin water transfer scheme. It was found that this water plays an important role in sustaining streamflow in the lower part of the catchment before discharging back into the Kafue River. It was thus recommended that efforts to improve irrigation efficiency in the lower catchment must simultaneously ensure downstream flows are maintained. Based on the findings of the past and present interactions, it was noted that irrigated agriculture had two contrasting effects on dry season flows depending on the source of irrigation water. In the upper and middle catchment where irrigation water was sourced from the Kaleya River, irrigation reduced dry season flows despite some contributions from return flows. In the downstream part of the catchment where irrigation water comes from the neighbouring Kafue River (intra-basin transfer), irrigation increased dry season flows through return flow contributions to the lower Kaleya River. Having better understood the climatic-landscape-hydrological interactions of the past and present, the potential future changes in climate and their effects on blue water flow (streamflow), green water flow (evapotranspiration – ET) and sediment load were evaluated. This was aimed at getting a holistic overview of the interactions so that management interventions could anticipate the future changes as this is necessary for long-lasting beneficial effects. Two Global Climate Models (GCMs) [MICROC5 and MPI-ESM-LR] and an ensemble (mean) dataset from five GCMs that had the highest Nash Sutcliffe Efficiency (≥ 0.29) and Heidke skill score (≥ 85%) for the Kaleya River Catchment were used to account for uncertainties in GCMs. The Soil and Water Assessment Tool (SWAT) hydrological model was calibrated and applied stochastically (to account for parameter uncertainty) and used to evaluate impacts of climate change on streamflow, ET, and sediment load. The period 1970 – 2005 was used as the baseline, while 2021 – 2050 was the future. Results based on the ensemble (mean) predicted a 6% and 12% increase in annual rainfall and a 1˚C and 2˚C increase in temperature compared to the baseline under the RCP 4.5 and RCP 8.5 scenarios, respectively. These changes were also accompanied by predicted increase in rainfall intensities. It was further predicted that maximum one-day rainfall would increase by 3% and 20% under the RCP 4.5 and RCP 8.5 scenarios, respectively. Additionally, the GCMs generally predicted increased number of Consecutive Dry Days (dry spell length) by about 2%–10% over the baseline. Taking the median (M95PPU – defined as the 50% uncertainty level for the hydrological model), and the GCMs ensemble mean climate, a 31% (9,675 m3 day -1) increase in annual streamflow was predicted under the RCP 8.5, accompanied by a sediment load increase of 144% (2,175 tonnes year-1) over the baseline. For the RCP 4.5 scenario, streamflow was predicted to increase by 21% (4,523 m3 day -1), accompanied by sediment load increase of 65% (994 tonnes year -1). With respect to green water flows, there was a predicted 2% (9mm) increase in annual ET under the RCP 4.5 scenario, and no change under the RCP 8.5 scenario. While climate change was predicted to increase water availability in both the rainy and dry seasons, landcover change could reverse the potential blue water gains in the dry season and reduce green water storage by about 13%. Further, the study evaluated the efficiency of Nature-based Solutions (NbSs) for managing increased rainfall intensities and the predicted increase in rainy season surface runoff and sediment load under different climate change scenarios. The NbSs virtual experiments were conducted using SWAT in SWAT-CUP. The reforestation NbS predicted the largest reductions in surface blue water (surface runoff) by 74% under the historical climate, 69% under the RCP 4.5 and 62% under the RCP 8.5 climate scenarios. Reforestation further resulted in predicted increase in deep aquifer recharge by 39% (historical), 26% (RCP 8.5 scenario) and 23% (RCP 4.5). Additionally, it was predicted that baseflow contribution to streamflow would increase by 11% (historical) and 2% (RCP 8.5) but not under the RCP 4.5 scenario (-2%). Green water flows (evapotranspiration) were predicted to increase by 3% (both RCP 4.5 and RCP 8.5%) and 2% (historical). Under the recharge structures NbS, it was predicted that surface runoff would reduce by about 2 - 4%, baseflow contribution to streamflow and deep aquifer recharge would increase by about 4%, without any change in ET under all climate scenarios. Conservation tillage NbS had a negligible predicted effect on water balance components at a catchment scale, suggesting that the water benefits could mainly be at a field scale. However, the effects of Conservation tillage on sediment load were noticeable even at a catchment scale. On sediment load, the highest change was predicted under the recharge structures NbS (-34% historical, -24% RCP 4.5 and -15% RCP 8.5 scenario), followed by reforestation (-15% historical, -7% RCP 4.5 scenario and -6% RCP 8.5 scenario) and conservation tillage (-4% historical, -2% RCP 4.5 and -1% RCP 8.5 scenario). From the green-blue water perspective, it was concluded that these nature-based solutions could assist in managing the increased rainfall and its intensities, and the ensuing high rainy season surface runoff and sediment load. The NbSs could thus assist in storing rainwater in the catchment for longer periods by converting it to deep groundwater, and baseflow and increasing the productive green water flows. The NbS could also be effective in sediment load management. In conclusion, the interactions of the changing landscape patterns with the changing climate and weather extremes and the effects on local green and blue water availability were investigated in this study. Overall, the study found that landscape pattern changes (patchiness of cover in addition to percentage of cover) amplify the negative effects of the changing climatic and weather extremes in the past, present, and future periods. But if well designed in form of NbS interventions, the landscape patterns could be used to manage the effects of climate change whereby the increasing rainfall intensities could be taken as a resource (not a case) for improving local water storage, and productivity. This could assist in building resilience to other climatic extremes such as dry spells, rainfall variability and increasing air temperatures.