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at the study site. The result of surface water availability analysis will be used as an input data of conservation infrastructure.
2.8 Water Demand
Water demand analysis include water demand for irrigation, domestic, non- domestic, industrial, livestock, and fisheries in the recent time and in the future. The
population and the land use change will determine the quantity of water demand, for human daily activities, industries, irrigation, fisheries and etc. To project the
population and land use changes precisely is very difficult. Many approaches can be done, one of them is an exponential approach. This method uses the assumption of
population growth and land use change in the percentage of each year are constant Direktorat Jenderal SDA, 2001.
2.9 Water Balance
In the hydrological cycle, the relationship between flow inflow and outflow in a watershed is called water balance Direktorat Pengairan dan Irigasi, 2006. From a
hydrological viewpoint, the first step of watershed management is to evaluate past, present, and proposed management practices on a watershed. Watershed water
balance refers to the balance between the inflow of water as precipitation and the outflow of water as evapotranspiration, ground water discharge, and surface flow.
Basically, watershed water balance is an accounting tool to keep track of the hydrological cycle of a watershed. When the watershed water balance concept is
used in conjunction with probability analysis, it can evaluate the hydrological, economic,and ecological feasibility of past, present, and potential activities on a
watershed Tate, 1995. Watershed water balance is best illustrated as an equation. The water
balance equation is the single most recognized equation in hydrology. A basic water balance equation for a watershed is showed with following equation Tate, 1995 :
P = ET + SF + GWD ± SMC ± GWS
with: P
= Precipitation ET = Evapotranspiration
SF = Stream flow GWD = Ground Water Discharge
SMC = Soil moisture Capacity GWS = Groundwater Storage
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The water balance equation should be modified for the condition of non- groundwater basin area. There are no ground water storage and ground water
discharge. Water is only stored in a soil layer and in the surface. Those will be replaced with soil water discharge and soil water storage. Water balance equation
with modification for a non-groundwater watershed is showed with following equation:
P = ET + SF + SWD ± SMC ± SS
with: P
= Precipitation ET = Evapotranspiration
SF = Stream flow SWD = Soil Water Discharge
SMC = Soil Moisture Capacity SS
= Soil Storage
2.10 Soil Erosion
Watershed damage is primarily caused by the erosion procces. Soil erosion is caused by the erosive forces of wind or water Pimentel, 2000. Erosion results in the
degradation of a soil‘s productivity in a number of ways: it reduces the efficiency of
plant nutrient use, damages seedlings, decreases plants‘ rooting depth, reduces the soil water-holding capacity, decreases its permeability, increases runoff, and reduces
its infiltration rate Troeh et al., 1991. In general, soil erosion composed three-step process. It starts with the
detachment of soil particles, continues with the transport of those particles, and ends with the deposition of soil particles in a new location. Bare soils soils that lack a
cover of living or dead plant biomass are highly susceptible to erosion, even on flat land. There are three main types of water- induced soil erosion: sheet, rill, and gully
O‘geen and Schwankl, 2006. Incorrect land use and processing may accelerate erosion, and will lead
reduce soil productivity. Erosion problems related with the planning of water resources. Erosion will increase sediment loads in the river system and changes the
hydro-morphological conditions. If the erosion is high, it will change the river hydrology element, such as the increasing of runoff and decreasing base flow
Direktorat Jenderal SDA, 2001.
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2.11 Watershed Conservation 2.11.1 The Purposes of Water Resources Conservation
