Hydrologic Cycle. Water Availabilty. Surface Water. Groundwater

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1 Hydrologic Cycle Hydrologic ydoogccyce cycle Surface Water Groundwater Water Availabilty 1

2 Hydrologic Cycle Constant movement of water above, on, and, below the earth s surface (Heath) Endless circulation of water below ocean, atmosphere, and land (Freeze and Cherry) Different scales: global, watershed, local transpiration (Heath, 1983) Major Components Precipitation Precipitation measured by rain gages at weather stations, expressed in terms of depth over time, e.g., in/yr, in/day, cm/hr, Evaporation and transpiration (evapotranspiration) evaporation refers to transfer of water from free-water surface while transpiration from growing plants to the atmosphere Infiltration to groundwater infiltration capacity depends on vegetation, topography, and soil conditions Runoff and baseflow overland flow or depression storage when precipitation exceeds infiltration runoff is total flow in a stream channel baseflow is contribution from groundwater Measured with rain gauges Data from different gauges in a basin allows to create contour maps of lines of equal precipitation (isohyets) 2

3 Isohyets: Interpolation method Thiessen polygons 880 mm An isohyet or isohyetal line is a line joining points of equal precipitation on a map. A map with isohyets is called an isohyetal map Evaporation Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere Endothermic or Exothermic process? Condensation is the opposite process dew point is the T at which condensation occur Measured with shallow pans Evaporation Influential factors Wind T Humidity 3

4 Transpiration Plants transfer water from the ground to the atmosphere Seasonal process Factors T Humidity Wind Soil-moisture availability Type of plant Evapotranspiration Comprehensive term to include water losses from transpiration and and evaporation in a basin Potential ET: assuming no water deficiency for plants maximum Actual ET: assuming field conditions PET can be calculated from formulas (e.g., Thornthwaite & Mather, 1955, 1957) Evapotranspiration ET can be measured with a Lysimeter The lysimeter is a big 'plant pot' which is weighed. Additionally, the outflow at the bottom is measured. From this information precipitation and evapotranspiration values can be calculated by applying the water balance equation scale Infiltration Infiltration is the process of water entering the soil Factors: Precipitation Soil type Infiltration capacity Soil saturation Land cover Slope ET 4

5 Infiltration rate vs. Precipitation If precipitation rate < equilibrium infiltration capacity then all precipitation will infiltrate If precipitation rate > equilibrium infiltration capacity but < initial infiltration capacity, then some precipitation will infiltrate and then runoff will occur If precipitation rate > initial infiltration capacity then only runoff will occur Measuring water cycle s components 13: Wind speed, air temperature, CO2 concentration 1: Sun shine duration, air temperature, dew point and relative humidity 2: Wind speed and direction 7: Snow 8: sun radiation height 10: soil heat flux 3: precipitation 4: Lysimeter ET 9: diffuse radiation 5: precipitation 6: precipitation 12: soil moisture 12: soil T and tensiometer Groundwater and surface water: A single resource Groundwater-Surface Water Interaction (Winter et al., 1999) (Winter et al., 1999) 5

6 Groundwater-Surface Water Interaction II 6