CEHY0322 Hydrology Material 2 Water Balance PDF

Hydrology Material 2 Water Balance Hydrologic Cycle Hydrologic cycle with global annual average water balance given in units relative to a value of 100 for the rate of precipitation on land (from Chow et al. 1988)). Hydrological models try to schematize the dynamics of water f...

Hydrology Material 2 Water Balance Hydrologic Cycle Hydrologic cycle with global annual average water balance given in units relative to a value of 100 for the rate of precipitation on land (from Chow et al. 1988)). Hydrological models try to schematize the dynamics of water flow within the water cycle. Therefore, they deal with transfer of mass which may take place in different phases (solid, liquid, gas). Such mass transfers occur through exchanges of energy. Water Balance Estimation It is a tool to assess the current status and trends in water resources availability in an area over a specific period of time. Simplest Form of Water Balance P = Q + E ± ∆S where: P: Precipitation Q: Runoff E: Evaporation ∆S: Storage in the soil, aquifers or reservoirs Example of water balance estimation. Source: BGR (2008) CEHY0322 Hydrology Material 2 Water Balance Conservation of Water Precipitation (rainwater) is either transferred to surface run off or evaporates OR it is stored in the ground. Use of Water Balance 1. Asses the current status and trends in water resource availability in an area over a specific period of time. 2. Strengthen water management decision-making by assessing and improving the validity of visions, scenarios and strategies. Problems occurring in water balance estimation: Temporal and spatial boundaries are not defined Quality of input data is poor Double counting of water flows when water flows within an area added to water flow of exiting area Inappropriate extrapolation of field level information to a larger scale Intuition (often based on popular myths) is used rather than good quality information. The storage term(s) of the water balance is omitted Political or other pressures result in unreliable estimates that have been manipulated Water balance estimation as basis for: Quantitative evaluation of water resources and their change under human activities Hydrological substantiation of projects for the rational use, control and redistribution of water resources in time and space Calculating Water Balance Simplest Form P + (Qin – Qout) – E = Δs P: Precipitation Σin – Σout = Δs Q: Discharge E: Evaporation ∆S: Storage in the soil, aquifers or reservoir CEHY0322 Hydrology Material 2 Water Balance A conceptual mathematical model for overall budget over the basin is: P – R – G – E – T = ∆S P: Precipitation R: Runoff G: Groundwater flow E: Evaporation T: Transpiration ∆S: Storage in the soil, aquifers or reservoirs A water shed: Sample Problems 1. A lake has an area of 10 km2. During a specific month, the lake evaporation was 90mm. During the same month, the inflow to the lake from a river was on average 1.2 m3/s and the outflow from the lake via another river was on average 1.1 m3/s. Also, for the same month, a water level increase of 100 mm for the lake was observed. What was the precipitation in mm during that month? 2. The average rainfall in a watershed is 50 in/year, and the average evapotranspiration is 32 in/yr. Assume no ground water crosses the watershed boundary. What is the stream runoff flux? What fraction of rainfall is evapotranspiration? What fraction of rainfall is runoff flux? CEHY0322 Hydrology Material 2 Water Balance