The two most important natural resources needed for agricultural development are land and water. The effective use and management of these resources will impact crop productivity. Simulation and prediction of rainfall and associated river flows are important in water resources planning and management (Lampurlanés et al., 2016). Any hydrological investigation, such as rainfall and streamflow estimates, is the first step in designing water-related research, requiring the measurement and collection of hydrological and meteorological data. However, like other developing countries, Ethiopia lacks stations to measure the quality and quantity of hydrometeorological data (Yitbarek et al., 2022), and according to information from experts, some of the stations are not functioning properly because there aren't enough skilled technicians to operate and maintain the current instruments (Halwatura & Najim, 2013). Therefore, an alternative hydrological model is needed. To construct soil and water conservation structures, an understanding of hydrological phenomena is required, such as flow fluctuations with changes in climatic, geographical or physical parameters(J.C. Refsgaard, 1990). When constructing and planning soil and water conservation structures, it is preferable to estimate the stream flows with regard to their frequency and amplitude (Sentis, 2010). Runoff can result from excessive rainfall when the amount of water released exceeds the capacity of a river, lake or man-made reservoir.Water levels in rivers and lakes under various boundary conditions are often modeled or predicted using rainfall-runoff models (J.C. Refsgaard, 1990).
In water science, hydrological models can be clustered, semidistributed, distributed, steady, unsteady, deterministic, or stochastic. The priority of the models is determined by the characteristics of the river basin, the research purpose and the hydrological forecasting purpose. Hydrological models are used to determine the hydrological response of a river basin to rainfall. In general, hydrological models are simplified simulations of complex hydrological systems (Diriba, 2021; Melesse & Abtew, 2015) and can classify in different ways, but the major classification is as a Physical model which represents the scaled-down representation of the real-world system in the laboratory for example simulation of open channel flow. HEC-HMS (Hydrologic Engineering Center - Hydrologic Modeling System, U.S. Army Corps of Engineers) is one of the hydrologic models used to simulate rainfall-runoff processes and routing (HEC HMS, 2000). The HEC model represents a watershed with integrated hydrologic and hydraulic components to simulate the response of watershed surface runoff to precipitation. Many studies demonstrate the ability of the HEC-HMS model to simulate and predict flow rates and its relevance in different geographical contexts (Wang et al., 2016). Loss, variability, and baseflow are the three main processes included in the HEC-HMS basin model.Within a portion of a river basin or catchment called a sub-basin, each component of the model performs a specific function of the rainfall flow process (Scharffenberg, 2013).
The model includes several components to address precipitation, streamflow, and layout. For example, the HEC-HMS model has been widely used in many hydrological studies due to its simplicity and usability in conventional methods (Halwatura & Najim, 2013). Many scientists have conducted important hydrological studies using the HEC-HMS model, which has demonstrated its ability to simulate and predict streamflow. It is applicable in many different geographical areas such as arid environments, tropical river basins, semi-arid regions; river basins in small areas, etc. to solve the biggest possible problems, used HEC-HMS for rainfall-runoff simulation(Gebre, 2015; Laouacheria & Mansouri, 2015). HEC-HMS model has been also used to simulate rainfall-runoff process with geo-informatics and atmospheric models for flood forecasting and early warnings in different regions of the world(Halwatura & Najim, 2013; Khaddor et al., 2015).
The HEC-GeoHMS (Geospatial Hydrologic Model Extension) is a public domain software package for use with GIS (geographic information system), GeoHMS ArcView, and Spatial Analyst to develop multiple model inputs hydrographic pattern. After analyzing digital elevation model (DEM) information, HEC-GeoHMS converts drainage routes and basin boundaries into hydrological data structures that represent the basin's response to rainfall (Hoogestraat, 2011). An important feature of the HEC-GeoHMS model is the assignment of the curve numbers (CN), which are related to the land use/land cover and soil type of the Gelana catchment. This study's major goal is to create the HEC-HMS hydrological model employing hydrological, meteorological, and remotely sensed data. The Gelana watershed's rainfall-runoff processes were simulated using the hydrological model in conjunction with HEC-GeoHMS and GIS to determine the flow. The HEC-HMS model's input parameters were computed. Study area
The Gelana catchment is situated in the rift valley lakes' Abaya-Chamo sub-basin. The catchment is located in southern Ethiopia. Geographically, the watershed lies between latitudes 50 25° and 6°18′ N and longitudes 370 50° and 380°E. The existing stream is the Gelana River, which flows south of the Yirga Chefe (upper valley) and passes through a small gorge before dropping into Lake Abaya. Due to poor soil, uneven terrain, and extensive forested areas around the Gelana and Jelo rivers, the Gelana basin was not included (Final feasibility study 2007 Gelana Irrigation project, 2007). The traditional classification of the country's climate based on altitude and temperature suggests the presence of five climatic zones, including Wurch (cold climate at altitudes above 3000 m above sea level), Dega (temperate climate like highlands with altitude 2500–3000 m), Woina Dega (hot 1,500-2,500 m above sea level), Kola (hot and arid type, below 1,500 m above sea level) and Bereha (hot and super dry type) (NMSA, 2001). As a result, the Gelana's categorization in the watershed spans from semi-arid in the lowland to humid in the highland. The rain surrounding Gelana Catchment exhibits a bimodal pattern, with maximum precipitation above 2649.1 mm/year in the highlands area during the months of April, May, and October, while precipitation is frequently below 515.6 mm/year at the lowest height in the low land. In the lowland region of the Gelana watershed, the average monthly maximum temperature ranges from 20.90C to 28.090C, and in the catchment's high land, it ranges from 8.250C to 13.540C.