Chapter 5 Hydrology

• Meteorological factors. If natural evaporation is viewed as an energy-exchange process, it can be demonstrated that radiation is by far the most important single factor and that the term solar evaporation is basically applicable.
• Nature of evaporating surface. All surfaces exposed to precipitation, such as vegetation, buildings, and paved streets, are potentially evaporation surfaces.
• Rate of evaporation from soil surfaces is limited by the availability of water, or evaporation opportunity.
• Effects of water quality. The effect of salinity, or dissolved solids, is brought about by the reduced vapor pressure of the solution.
• Seepage is usually the most difficult factor to evaluate since it must be estimated indirectly from measurements of groundwater levels, permeability, etc.
• Water-stage recorders are sufficiently precise for determining the storage changes provided that the stage-area relationship is accurately established.
• Expansion or contraction of stored water with large temperature changes can introduce appreciable errors.
• The energy-budget approach, like the water budget, employs a continuity equation and solves for evaporation as the residual required to maintain a balance.
• Sensible-heat transfer cannot readily be observed or computed, and the Bowen ratio was conceived as a means of eliminating this term from the energy-budget equation.
• Radiometers can be designed to measure either the total incoming or net radiation.
• Net radiometers must be exposed over the water at one or more points constituting a representative surface temperature.
• Another approach to the determination of net radiation involves the application of the energy budget to an insulated evaporation pan. (Radiation Integrator)
• The theoretical development of turbulent-transport equations has followed two basic approaches: the discontinuous or mixing-length, concept introduced by Prandtl and Schmidt, and the continuous-mixing concept of Taylor.
• Although experimental data are insufficient to determine the magnitude of the size effect, it is believed that Eq. 5-12 can be applied to lakes ranging up to several hundred square kilometers without appreciable error in this respect if all observations are well centered in the lake.
• The pan is undoubtedly the most widely used evaporation instrument today, and its application in hydrologic design and operation is of long standing.
• Although criticism of the pan may be justified on theoretical grounds, for some types of pans the ratio of annual lake-to-pan evaporation (pan coefficient) is quite consistent, year by year, and does not vary excessively from region to region.
• Pan observations. There are three types of exposures employed for pan installations--- sunken, floating, and surface and divergent views on the best exposure persist.
• Sunken pans collect more trash; they are difficult to install, clean, and repair; leaks are not easily detected; and height of vegetation adjacent to the pan is quite critical.
• Pan evaporation and meteorological factors. Many attempts have been made to derive reliable relations between pan evaporation and meteorological factors.
• Obvious purposes to be served by such relations are as follows:

  • To increase our knowledge of evaporation.
  • To estimate missing pan records
  • To estimate data for stations at which pan observations are not made.
  • To test the reliability and representativeness of observed data.
  • To aid the study of lake-pan relations.
• Pan coefficients. Water-budget, energy-budget, and aerodynamic techniques can be used to estimate evaporation from existing reservoirs and lakes.
• Effects of advected energy on pan evaporation. Observations demonstrate that the sensible-heat transfer through the pan can be appreciable and may flow in either direction, depending upon water and air temperatures.
• In reservoir design, the engineer is really concerned with the increased loss over the reservoir site resulting from the construction of the dam, i.e., reservoir evaporation less evapotranspiration under natural conditions.
• In humid areas construction of a dam causes only a nominal increase in water loss.
• Small reservoirs are sometimes entirely covered to reduce evaporation losses.
• Only minute portions of the water absorbed by the root systems of plants remain in the plant tissues; virtually all is discharged to the atmosphere as vapor through transpiration.
• Once inside the root, the water is transferred through the plant to the intercellular space within the leaves.
• Air enters the leaf through the stomata, openings in the leaf surface, and the chloroplasts within the leaf use carbon dioxide from the air and a small portion of the available water to manufacture carbohydrates for plant growth (photosynthesis).
• As air enters the leaf, water escapes through the open stomata; this is the process of transpiration.
• Some investigators believe that transpiration is independent of available moisture until it has receded to the wilting point (moisture content at which permanent wilting of plants occurs), while others assume that transpiration is roughly proportional to the moisture remaining in the soil and available to the plants.
• Plant type becomes an important factor in controlling transpiration when available soil moisture is limited.
• The relative transpiration is not proportional to cover density, however, for two reasons : (1) an isolated plant receives radiation on the side facing the sun which would fall on an adjacent plant were there solid cover, and (2) a portion of the radiation reaching the ground is subsequently transmitted to the plants (oasis effect).
• Xerophytes, desert species, which have fewer stomata per unit area and less surface area exposed to radiation, transpire relatively little water.
• Phreatophytes, on the other hand, have root systems reaching to the water table and transpire at rates largely independent of moisture content in the zone of aeration.
• All plants can control stomatal opening to some extent, and thus even mesophytes, plants of the temperate zones, have some ability to reduce transpiration during periods of drought.
• Even aquatic plants, hydrophytes, cannot pump water into the atmosphere at rates in excess of those controlled by available radiant and sensible energy.
• Most measurements are made with a phytometer, a large vessel filled with soil in which one or more plants are rooted.
• Ceramic and Piche atmometers have been widely used in transpiration studies. Such instruments automatically feed water from a reservoir to an exposed, wetted surface.
• In studying the hydrologic balance for a catchment area, one is usually concerned only with the total evaporation (or evapotranspiration), the evaporation from all water, soil, snow, ice, vegetation, and the other surfaces plus transpiration.
• Consumptive use is the total evaporation from an area plus the water used directly in building plant tissue.