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Understanding the Hydrologic Cycle: Watershed Management

This chapter provides an overview of the hydrologic cycle and its importance in watershed management. It covers topics such as water in the atmosphere, interception, infiltration, runoff, and factors affecting runoff. It also discusses the design runoff rate and methods for measuring and predicting runoff.

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Understanding the Hydrologic Cycle: Watershed Management

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  1. 2.4 hydrology 2.4.1 Hydrologic cycle Watershed Management--8

  2. Water in atmosphere output input E T E Interception Channel storage Plant storage Overland flow Surface water Subsurface flow To plants Soil water To plants Seepage Groundwater Streamflow output T H E H Y D R O L O G I C CY C L E

  3. Chapter 2  The watershed management inventory 2.4.2 Interception Ic = Pg – Th – Sf Where Ic = canopy interception (mm) Pg = gross precipitation (mm) Th = throughfall (mm) Sf = stemflow (mm) Watershed Management--8

  4. Measurement of interception Ic = Pg – Th – Sf Raingauge out of the tree (Pg) Raingauge under the tree (Th) Steam collar (Sf) Watershed Management--8

  5. 2.4.3 Infiltration Watershed Management--8

  6. Measurement of infiltration • Double-ring infiltrometer is used in which one cylinder with 30cm in diameter is placed inside another with 46-50cm in diameter. • Rainfall-runoff plot-type infiltrometer Watershed Management--8

  7. 2.4.4 Runoff Definition Runoff is that portion of the precipitation that makes its way toward stream channels, lakes, or oceans as surface or subsurface flow. However this term usually means surface flow. Watershed Management--8

  8. Factors affecting runoff • Rainfall-- duration, intensity, and areal distribution influence the rate and volume of runoff. Watershed Management--8

  9. Factors affecting runoff Size--Both runoff volumes and rates increase as watershed size increases; however both rate and volume per unit of watershed area decrease as the runoff area increases. • Shape--Long narrow watersheds are likely to have lower runoff rates than more compact watersheds of the same size. Watershed Management--8

  10. Factors affecting runoff Orientation-- Storms moving upstream cause a lower peak runoff rate than storms moving downstream. • Topography--Land slope, gradients of channels, depressed areas affect rates and volumes of runoff. Watershed Management--8

  11. Factors affecting runoff • Geology--The geologic or soil materials determine to a large degree the infiltration rate, and thus affect runoff. • Surface--Vegetation and the practices incident to agriculture and forestry also influence infiltration. Structures such as dams, levees, bridges,and culverts all influence runoff rates. Watershed Management--8

  12. Design runoff rate • The capacity to be provided in a structure that must carry runoff may be termed the design runoff rate. • Vegetated controls and temporary structures are usually designed for a runoff that may be expected to occur once in 10 once in 50 or 100 years. Watershed Management--8

  13. Design runoff rate • Expensive, permanent structure will be designed for runoffs expected only once in 50 or 100 years. • Designed return period, also called recurrence interval, depends on the economic balance between the cost of periodic repair or replacement of the facility and the cost of providing additional capacity to reduce the frequency of repair or replacement. Watershed Management--8

  14. Design runoff rate • Downstream damage potentially resulting from failure of the structure may dictate the choice of the design frequency. Watershed Management--8

  15. Design runoff rate The ratio method of predicting a design peak runoff rate is expressed by the equation q = 0.0028CiA Where q = the design peak runoff rate in m3/s C = the runoff coefficient i = rainfall intensity in mm/h for the design return period and for a duration equal to the “time of concentration” of the watershed A = the watershed area in ha Watershed Management--8

  16. Design runoff rate • Time of concentration of a watershed is the time required for water to flow from the most remote (in time of flow) point of the area to the outlet once the soil has become saturated and minor depressions filled. Tc = 0.0195L0.77Sg-0.385 Where Tc= time of concentration in min L = maximum length of flow in m Sg= the watershed gradient in m/m or the difference in elevation between the outlet and the most remote point divided by the length, L. Watershed Management--8

  17. Design runoff rate Soil Conservation Service method q = quAQ Where q = peak runoff rate (m3/s) qu= unit peak flow rate (m3/s per ha/mm of runoff) A = watershed area in ha Q = runoff depth in mm Watershed Management--8

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