Topography Impacts Hydrology in the Sub-Humid Ethiopian Highlands

Understanding the relationship between topography, hydrological processes, and runoff source areas is essential in engineering design, such as predicting floods and implementing effective watershed management practices. This relationship is not well defined in the highlands with a monsoon climate and needs further study. The objective of this study is to relate topographic position and hydrological response in tropical highlands. The research was conducted in the Debre Mawi watershed in the northwest sub-humid Ethiopian highlands. In the monsoon rain phase of 2017 and 2018, groundwater depth, infiltration rate, and surface runoff were monitored at the upslope, midslope, and downslope positions. Surface runoff rates were measured in farmer fields through distributed V-notch weirs as estimates of positional runoff. Average water table depths were 30 cm deep in the downslope regions and 95 cm in the upslope position. The water table depth affected the steady-state infiltration rate in the rain phase. It was high upslope (350 mm h−1), low midslope (49 mm h−1), and zero downslope. In 2017, the average runoff coefficients were 0.29 for the upslope and midslope and 0.73 downslope. Thus, topographic position affects all aspects of the watershed hydrology in the humid highlands and is critical in determining runoff response.

Kajian Peran Stakeholder Pada Implementasi Kebijakan Pengelolaan DAS Terpadu, Studi Kasus DAS Krueng Aceh

Kerusakan daerah aliran sungai (DAS) semakin meningkat, disamping karena faktor perubahan tutupan lahan, faktor pengelolaan DAS juga menjadi pemicu. Peraturan Pemerintah (PP) No.37 Tahun 2012 dan Perda (Qanun) No. 7 tahun 2018 merupakan landasan pengelolaan DAS di Aceh. DAS Krueng Aceh merupakan DAS prioritas yang kondisinya kritis, pengelolaan DAS Krueng Aceh memerlukan penanganan terpadu oleh stakeholders terkait. Kajian ini bertujuan untuk menganalisis PP, Qanun, stakeholders pengelola DAS Krueng Aceh dan implementasi kebijakan berdasarkan perubahan kondisi biofisik DAS. Analisis yang digunakan kajian ini adalah analisis perubahan kondisi biofisik DAS, analisis stakeholders, serta analisis implementasi kebijakan. Berdasarkan hasil analisis kondisi biofisik DAS, tutupan lahan DAS Krueng Aceh dalam 10 tahun terakhir menunjukkan perubahan yang sangat signifikan pada tahun 2020, perubahan ini berdampak pada kondisi hidrologi DAS dimana terjadi perubahan debit sungai maksimum pada tahun 2020 sebesar 15.78 m³/detik dibandingkan tahun 2019 sebesar 10.09 m³/detik. Dari pendalaman isi PP dan Qanun diketahui hal pokok kegiatan pengelolaan DAS yakni ; peningkatan daya dukung DAS, pengelolaan sumber daya air, dan penataan ruang. Terdapat 20 stakeholders yang terlibat pengelolaan DAS Krueng Aceh, stakeholders pemerintah memiliki kepentingan yang tinggi dan sumber daya untuk melaksanakan kegiatan. Bila dilihat dari kondisi biofisik DAS tahun 2020 dan dikaitkan dengan peraturan perundangan, analisis stakeholders, serta analisis implementasi kebijakan dapat dikatakan bahwa kegiatan pengelolaan DAS secara terpadu belum berjalan sesuai isi kebijakan. Untuk mewujudkan hal tersebut perintah PP dan Qanun harus dilaksanakan secara tegas, Tim Koordinasi Pengelolaan DAS Terpadu (TKPDAS-T) yang sudah dibentuk harus sesegera mungkin difungsikan untuk mewujudkan implementasi kebijakan pengelolaan DAS terpadu.ABSTRACTWatershed damage is increasing, in addition to changes in land cover, watershed management factors are also a trigger. Government Regulation (PP) No.37 of 2012 and Perda (Qanun) No. 7 of 2018 is the cornerstone of watershed management in Aceh. Krueng Aceh watershed is a priority watershed whose condition is critical, the management of Krueng Aceh watershed requires integrated handling by relevant stakeholders. This study aims to analyze PP, Qanun, stakeholders of Krueng Aceh watershed management and policy implementation based on changes in watershed biophysical conditions. The analysis used by this study is an analysis of changes in the biophysical condition of the watershed, stakeholder analysis, and policy implementation analysis. Based on the results of the analysis of the biophysical condition of the watershed, the cover of the Krueng Aceh watershed in the last 10 years showed a very significant change in 2020, this change has an impact on the condition of watershed hydrology where there is a maximum river discharge change in 2020 of 15.78 m³ / second compared to 2019 of 10.09 m³ / second. From the deepening of the contents of PP and Qanun, it is known that the main things of watershed management activities are; increased watershed carrying capacity, water resource management, and spatial arrangement. There are 20 stakeholders involved in the management of the Krueng Aceh watershed, government stakeholders have high interests and resources to carry out activities. When viewed from the biophysical condition of the watershed in 2020 and associated with legislation, stakeholder analysis, and policy implementation analysis, it can be said that integrated watershed management activities have not been run with the contents of the policy. To realize this, the PP and Qanun orders must be implemented strictly, the Integrated Watershed Management Coordination Team (TKPDAS-T) that has been established must be as soon as possible to realize the implementation of integrated watershed management policies.

Flow Indices Variability in Humid Subtropical of Upper Awash River Basin, Ethiopia

Investigating the hydrological extremes indices at high resolutions describing the whole stream spectrum is essential for the comprehensive assessment of watershed hydrology. The study focuses on a wide-ranging assessment of river discharge in annual mean, peak, and high and low percentiles flow at the Upper Awash River basin, Ethiopia. Statistical tests such as coefficient of variation, flood variability to characterize the flow regime and Tukey’s test to detect decadal variability. Modified Mann-Kendall test, Sen’s slope estimator, innovative trend analysis and Pettitt’s test were applied to see trends, and change points in time series, respectively. Results showed that the basin was characterized by moderate to high variability. Spatially, main tributaries showed a higher variability, almost in all-time step and characterized by higher flood variability. The large discharge receiving rivers resulted in a moderate to high and lower discharge variability. Test statistics resulted in a positive increasing trend dominating most time scales at a 5% significant level and higher magnitude of slope trend in peak flow. A negative trends were also exhibited. Hombole main outlet site experienced decreasing trend in high percentile flow. In comparison, complete trend direction agreements were observed (except in few series). Flow indices showed an upward shift and downward shift mainly in the year 2000s and the significant decadal variation resulted in comparable with change points. The study provides an understanding of water resources variability, which will be necessary to apply operational water resources strategies and management to restrain the potential impacts of variability nature of the streamflow.

GIS & Remote Sensing Based Morphometric Parameters and Topographic Changes of the Lower Orashi River in Niger Delta

In watershed hydrology, the morphometric features of a river basin are vital to examine the lower Orashi River basin morphological and hydrological aspects, as well as its flood potential, based on their morphometric characteristics using remotely sensed SRTM data that was analyzed with ArcGIS software. The areal, linear, and relief aspects of the Orashi River basin were examined as morphometric parameters. The lower Orashi river basin, according to the findings, has a total size of 625.61 km2 and a perimeter of 307.98 km, with a 5th order river network based on Strahler categorization and a dendritic drainage pattern. Because of low drainage density, the drainage texture is very fine, the relief is low, and the slope is very low. Bifurcation ratio, circularity ratio, drainage density aspect ratio, form factor, and stream frequency values indicate that the basin is less elongated and would produce surface runoff for a longer period, while topographic changes show that the river is decreasing with depth in the land area at about the same elevation as a result of sand deposited due to lack of maintenance by dredging, which implies that the basin is morphometrically elevated and sensitive to erosion and flooding. To understand geohydrological features and to plan and manage watersheds, morphometric analysis based on geographic information systems and remote sensing techniques is beneficial.

Modeled forest conversion influences humid tropical watershed hydrology more than projected climate change

In the humid tropics, forest conversion and climate change threaten the hydrological function and stationarity of watersheds, particularly in steep terrain. As climate change intensifies, shifting precipitation patterns and expanding agricultural and pastoral land use may effectively reduce the resilience of headwater catchments. Compounding this problem is the limited long-term monitoring in developing countries for planning in an uncertain future. In this paper, we asked which change, climate or land use, more greatly affects stream discharge in humid tropical mountain watersheds? To answer this question, we used the process-based, spatially distributed Soil Moisture Routing model. After first evaluating model performance (Ns = 0.73), we conducted a global sensitivity analysis to identify the model parameters that most strongly influence simulated watershed discharge. In particular, peak flows are most influenced by input model parameters that represent baseflow and shallow subsurface soil pathways while low flows are most sensitive to antecedent moisture, macropore hydraulic conductivity, soil depth and porosity parameters. We then simulated a range of land use and climate scenarios in three mountain watersheds of central Costa Rica. Our results show that deforestation influences streamflow more than altered precipitation and temperature patterns through changes in first-order hydrologic hillslope processes. However, forest conversion coupled with intensifying precipitation events amplifies hydrological extremes, reducing the hydrological resilience to predicted climate shifts in mountain watersheds of the humid tropics. This finding suggests that reforestation can help mitigate the effects of climate change on streamflow dynamics in the tropics including impacts to water availability, flood pulses, channel geomorphology and aquatic habitat associated with altered flow regimes.

Assessment of Long-term Groundwater Use Increase and Forest Growth Impact on Watershed Hydrology

This study used Soil and Water Assessment Tool (SWAT) to investigate the impacts of groundwater use increase and forest growth on the watershed hydrology of Geum River basin (9,645.5 km2), South Korea. Groundwater use increase and forest growth data from 1976 to 2015 were prepared in 10-year interval and were reflected to SWAT corresponding to each decade. SWAT was calibrated in the aspect of evapotranspiration, soil moisture, and streamflow using the observation data. The model performance for streamflow was evaluated by coefficient of determination (R2), Nash-Sutcliffe efficiency (NSE), root mean square error (RMSE), and percent bias (PBIAS). The calibration achieved the average R2 value of 0.73 ~ 0.82, NSE value of 0.75 ~ 0.81, RMSE value of 0.53 ~ 2.35 mm/day, and PBIAS value of -2.51 ~ + 11.74%, respectively. The model performance for evapotranspiration and soil moisture was evaluated by R2. The calibration result of evapotranspiration and soil moisture achieved average R2 value of 0.45 and 0.44, respectively. The calibrated model evaluated the impact of two factors on watershed hydrology. Decadal increase of groundwater use has decreased groundwater flow and increased groundwater recharge while decadal forest growth has mainly increased evapotranspiration that led to the decrease of other hydrological components. Resultingly, the change of two factors have imposed temporal decrease of total runoff on the watershed while the influence of two factors on annual streamflow loss was bigger in lower flow rate.

Projecting the impacts of end of century climate extremes on the hydrology in California

In California, it is essential to understand the evolution of water resources in response to a changing climate to sustain its economy and agriculture and build resilient communities. Although extreme conditions have characterized the historical hydroclimate of California, climate change will likely intensify hydroclimatic extremes by the End of Century (EoC). However, few studies have investigated the impacts of EoC extremes on watershed hydrology. We use cutting-edge global climate and integrated hydrologic models to simulate EoC extremes and their effects on the water-energy balance. We assess the impacts of projected driest, median, and wettest water years under a Representative Concentration Pathway (RCP) 8.5 on the hydrodynamics of the Cosumnes river basin. High temperatures (> 2.5 °C) and precipitation (> 38 %) will characterize the EoC extreme water years compared to their historical counterparts. Also, precipitation, mostly in the form of rain, is projected to fall earlier. This change reduces snowpack by more than 90 %, increases peak surface water and groundwater storages up to 75 % and 23 %, respectively, and makes these peak storages occur earlier in the year. Because EoC temperatures and soil moisture are high, both potential and actual evapotranspiration (ET) increase. The latter, along with the lack of snowmelt in the warm EoC, cause surface water and groundwater storages to significantly decrease in summer, with groundwater showing the highest rates of decrease. Besides, the changes in the precipitation phase lead the lower-order streams to dry out in EoC summer whereas the mainstream experiences an increase in storage.

Evaluation of the impacts of land use/cover changes on water balance of Bilate watershed, Rift valley basin, Ethiopia

Land use/cover change is one of the responsible factors for changing the water balance of the watershed by altering the magnitude of surface runoff, interflow, base flow, and evapotranspiration. This study was aimed at evaluating the impacts of land use/cover change on the water balance of Bilate watershed between 1989, 2002, and 2015. The water balance simulation model (WaSiM) was used to access the impacts of land use/cover change on water balance. The model was calibrated (1989–2003) and validated (2007–2015) using the streamflow of at Bilate Tena gauging station. The result of land-use dynamics showed land use/cover change has a significant impact on the water balance of the watershed like on runoff production, base flow, interflow, evapotranspiration, and total simulation flow. In the study watershed, the change in total simulated flow increased by 77.83%, and surface runoff, interflow, and base flow increased by 80.23%, 75.69%, and 87.79% respectively and evapotranspiration decreased by 6% throughout the study period (1989–2015). The results obtained from this study revealed that the watershed is under the land/cover change that shows its impacts on hydrological processes and water balance. Thus, effective information regarding the environmental response of land use/cover, change is important to hydrologists, land-use planners, watershed management, and decision-makers for sustainable water resource projects and ecosystem services. Therefore, the policy-makers, planners, and stakeholders should design strategies to ensure the sustainability of the watershed hydrology for the sake of protecting agricultural activities, and urban planning and management systems within the watershed area.