Hydrology is a branch of science that studies all matters of water contained on the surface of the earth, which involves the amount and quality, distribution according to time and space, and its movements follow a cycle known as the hydrological cycle. Since the beginning of the development of hydrology as a technology and science, hydrological research has always been based on human basic needs for water, from domestic water supply, flood and drought relief, to irrigation, electricity, transportation and recreation.
All hydrological processes take place in the watershed, so that the watershed is a natural laboratory of hydrology. According to the type of watershed, the hydrological processes vary according to space and time within the watershed boundary. Topography, soil, geology, vegetation, land use, and river networks are the main factors that contribute to variations in the hydrological processes. For hydrological analysis, a watershed is expressed as a spatial unit in which hydrological problems need to be resolved, so that the concept is known “one watershed one unit of management area”. In this sense it is commonly limited to watersheds as an area unit in the form of a land system with vegetation cover that is bounded by natural topographic boundaries (such as ridges) that receive rainfall, collect and store water, sediments and other nutrients, and drain them through tributaries to eventually exit through one main river into the sea or lake.As a physical system with its hydrological function, the watershed does not have to be an ecosystem whole (Pawitan, 1998).
Data needed for watershed modeling is hydrometeorology, geomorphology, agriculture, pedology, geology, and hydrology. Hydrometeorology data such as rainfall, snow, temperature, radiation, humidity, vapor pressure, hours of sunlight, wind speed, and evaporation. Agricultural data including vegetative cover, land use, treatment, and fertilizer application. Pedological data include soil type, texture and structure, soil conditions, soil particle size, porosity, water content and capillary pressure, infiltration steady state, saturated hydraulic conductivity, and humidity. Geological data including stratigraphic data, lithology, and structural control. More specifically, data on type, depth, and area. For limited data, including hydraulic conductivity, transmissivity, storativity, compressibility and porosity. For confined aquifer data, such as certain storage, hydraulic conductivity, porosity, watertable, and recharge area. Each data set must be checked with respect to homogeneity, completeness, and accuracy. Geomorphological data including topographic maps showing elevation contours, river networks, drainage areas, slopes and slope lengths, and watershed area. While hydrological data itself includes the depth of flow, river flow (streamflow discharge), base flow (base flow), interflow, river-aquifer interaction, potential, watertable, and drawdowns.