2. Lecture Notes_Hydrologic Cycle_Water Balance_24.pdf
- 1. Hydrologic Cycle And Water Balance
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- 3. • Water cycle is in balance, and the amount of precipitation falling will slowly soak into the ground and eventually reach the rivers • The hydrological cycle is a good example of a closed system: the total amount of water is the same, with virtually no water added to or lost from the cycle. • Water just moves from one storage type to another. • Water evaporating from the oceans is balanced by water being returned through precipitation and surface runoff. Hydrological Cycle
- 4. 4 • The Global hydrologic cycle is one of the most important natural cycles that operates on Earth. • It describes the circulation of water through the atmosphere, the land, and the oceans. • This cycle consists of set of storages (snow, soil moisture, and groundwater) and fluxes (precipitation, evapotranspiration, and runoff) that link the storages together. • Precipitation is the main driver of the water cycle and, on average, 70% of annual precipitation is lost due to evapotranspiration. Hydrologic Cycle
- 5. 1 Precipitation may be in the form of rain or snow. 2 Vegetation may intercept some fraction of precipitation. A large fraction of intercepted water is commonly evaporated back to the atmosphere. 3 How it Works : Language and Terminology Precipitation that penetrates the vegetation is referred to as throughfall and may consist of both precipitation that does not contact the vegetation, or that drops or drains off the vegetation after being intercepted. 5
- 6. 1 2 How it Works : Language and Terminology There is also flux of water to the atmosphere through transpiration of the vegetation and evaporation from soil and water bodies, Evapotranspiration The surface water input available for the generation of runoff consists of throughfall and snowmelt. Surface water input may accumulate on the surface in depression storage, or flow overland towards the streams as overland flow, or infiltrate into the soil, where it may flow laterally towards the stream contributing to interflow. 3 6
- 7. 1 2 3 How it Works : Language and Terminology 4 Infiltrated water may also percolate through deeper soil and rock layers into the groundwater. The water table is the surface below which the soil and rock is saturated and at pressure greater than atmospheric. Water table serves as the boundary between the saturated zone containing groundwater and unsaturated zone. Water added to the groundwater is referred to as ground water recharge. Immediately above the water table is a region of soil that is close to saturation, due to water being held by capillary forces. This is referred to as the capillary fringe. Lateral drainage of the groundwater into streams is referred to as baseflow, because it sustains streamflow during rainless periods. 7
- 8. 1 2 3 How it Works : Language and Terminology 4 Subsurface water, either from interflow or from groundwater may flow back across the land surface to add to overland flow. This is referred to as return flow. Overland flow and shallower interflow processes that transport water to the stream within the time scale of approximately a day or so are classified as runoff. The terms quick flow and delayed flow are also used to describe and distinguish between runoff and baseflow. Runoff includes surface runoff (overland flow) and subsurface runoff or subsurface stormflow (interflow). 8
- 9. Exercise Label the Rainfall-Runoff processes depicted in the figure 9
- 11. Water Balance • A water balance can be established for any area of earth’s surface by calculating the total precipitation input and the total of various outputs. • The water balance approach allows an examination of the hydrological cycle for any period of time. • The purpose of the water balance is to describe the various ways in which the water supply is expended. • The water balance is a method by which we can account for the hydrological cycle of a specific area, with emphasis on plants and soil moisture. 11
- 12. 12 “The mass in an isolated system can neither be created nor be destroyed but can be transformed from one form to another”
- 13. Water Balance • The water balance is defined by the general hydrologic cycle equation, which is basically a statement of the law of conservation of mass as applied to the hydrologic cycle. In its simplest form, this equation reads • Inflow - Outflow = Change in Storage Inflow = Outflow + Change in Storage • Water balance equations can be assessed for any area and for any period of time. 13
- 14. 14 The hydrologic cycle for a natural system is characterized by a water mass balance equation: Precipitation=Runoff + Infiltration + Evapotranspiration + ΔStorage. Water Balance Inflow - Outflow = Change in Storage Inflow = Outflow + Change in Storage
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- 16. What is the water balance/budget? In order to gain better understanding of water resources in a drainage basin, we use a simple equation called the water balance. The balance between water inputs and outputs of a drainage basin, may be shown as water budget graph-that is, the drainage basin operates as Water Budget Equation of a catchment for a time interval ∆t is written as: Input – Output = Change in Storage P – R – G- E- T = ∆S Where: P = Precipitation R = Surface runoff E = Evaporation T= Transpiration G = Net groundwater flow out of the catchment ∆S = Change in Storage . 16
- 18. • A water balance can be established for any area of earth’s surface by calculating the total precipitation input and the total of various outputs. • The water balance approach allows an examination of the hydrological cycle for any period of time. • The purpose of the water balance is to describe the various ways in which the water supply is expended. Applications of Water Balance Equation
- 19. • A region with an area of 1750 Km2 received 1250 mm of annual precipitation. Calculate the total amount of rainfall occurred in the year (in m3)
- 20. • A catchment with an area of 1750 Km2 received 1250 mm of annual precipitation. Calculate the total amount of rainfall occurred in the year (in m3) • Sol: Area of the catchment = 1750 Km2 = 1750 X 10^6 m2 • Precipitation received = 1250 mm = 1.25 m • Total annual precipitation = 1.25 X 1750X10^6 = 2187.5 X 10^6 m3
- 21. Example Problem: Precipitation over an area = 157500 m3 Runoff volume = 54000 m3 Estimate the Losses occurred in the considered region ? 21
- 22. Example Problem: Precipitation over an area = 157500 m3 Runoff volume = 54000 m3 Losses = 157500 – 54000 = 103500 m3 22
- 23. In a given year, a catchment with an average area of 1750 km2 received 1250 mm of precipitation. The average rate of flow measured in a river draining the catchment was 25 m3/s. Estimate the Losses ?
- 24. In a given year, a catchment with an average area of 1750 km2 received 1250 mm of precipitation. The average rate of flow measured in a river draining the catchment was 25 m3/s. Estimate the total flow occurred in the year along with the runoff coefficient. Precipitation = 1.25 X 1750 X 10^6 = 2187.5 X 10^6 m3 Total rainfall = 2187.5 X 10^6 / (365 X 24 X 60 X 60) = 69.36 m3/s Total rate of flow = 25 m3/s Runoff coefficient = Actual flow / Total precipitation occurred (Runoff/Rainfall) Runoff coefficient = 25 / 69.36 = 0.36 (36%) Runoff of the catchment / Rainfall of the catchment = 36 % That is 36% of rainfall is converting into runoff and remaining 64% is losses.
- 25. 25 A lake has an area of 15 km2. Observation of Hydrological variables during a certain year : P = 700 mm/year Average Inflow, Q_in = 1.4 m3/s Average Outflow, Q_out = 1.6 m3/s Assume that there is no net water exchange between the lakes and the groundwater. Determine the evaporation during this year.
- 26. 26 Sum of Inflows to the lake – sum of Outflows from the lake = Change in the Volume (P + Q_In) - (E+ Q_out) = ∆S Assuming storage changes over one year as Zero , ∆S = 0 E = P + Q_in – Q_out Q_in = 1.40 X 3600 X 24 X 365 / 15 X 10^6 = 2943.4 mm Q_Out = 1.60 X 3600 X 24 X 365 / 15 X 10^6 = 3363.8 mm E = 700 + 2943.4 +3363.8 = 279.6 mm