Course: BFM 101 Biofuels Fundamentals

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The Water Cycle Continued

Plant cover can also influence evaporation. A dense plant canopy, such as a forest, provides shade and cooler soils than does a row crop, where gaps in the crop canopy exposing bare soil will be present for some period of time. Forest leaf litter or crop residues keep the soil cooler and disrupt the airflow over the soil, decreasing evaporation. Thus, the type of crop and crop management can influence the soil’s evaporative moisture loss.

Water is also lost by transpiration, the release of water vapor from plant parts, especially the leaves via stomates. Transpiration also cools plants and enables flow of nutrients and water from roots to shoots. The rate of transpiration is directly related to the evaporation of water molecules from plant surface, especially from the surface openings, or stoma, on leaves.  When considering the water cycle, and because of their similar processes, transpiration is coupled with evaporation and described as evapotranspiration.

Evapotranspiration (water lost through evaporation and transpiration) is driven by solar energy, which is affected by the season, latitude, cloud cover and the time of day. Evapotranspiration is affected by the amount of available moisture in the soil, humidity, wind speed, plant type, plant density, root depth and reflective land-surface characteristics. Except for precipitation, evapotranspiration is considered the most important factor in the hydrologic cycle.

The U.S. Geological Survey report on evapotranspiration and drought indicates that 40 percent of the annual average precipitation in the northeast and northwest U.S. is lost in evapotranspiration, while 100 percent is lost in the Southwest [7].

Evapotranspiration from vegetation, such as corn and soybeans, is measured by comparison to evaporation from a standard-sized open pan kept full of water (Table 1.1). By this pan method, various crops or canopy covers can be compared to one another based on their relation to the pan-evaporation rate [9].

Table 1.1.  Comparison of two crops at emergence and full cover as % water evaporation from a pan.

Infiltration and Runoff – Precipitation that doesn’t evaporate or run off quickly has time to infiltrate. Water that moistens the soil but does not infiltrate into the groundwater may return to the atmosphere by evaporation and transpiration. Coarse soils allow for greater infiltration while fine soils, such as silts and clays, increase runoff. Impervious surfaces, such as roads, buildings, driveways and parking lots, create runoff since water accumulates but cannot infiltrate. Deep infiltration can store water for centuries. Lakes and streams may either be recharged by groundwater or lose water to groundwater. Groundwater discharging to the surface may be experienced as cold water inflow to streams, swamps, lakes or other surface water.

The soil surface plays a major role in infiltration. Infiltration and runoff rates are affected by soil texture, structural stability, organic content, the kind and amount of swelling clays, soil depth and the presence of impervious soil layers [10]. Soil management and land use in turn affect the infiltration capacity of the native soil. Plants break the impact of rain, slow the water movement and allow longer residence time so infiltration can occur. Mulches, leaf litter, and plant stubble: all can slow the water movement and allow for greater infiltration. Soil channels created by worms, wildlife, such as gophers, acids on limestone bedrock and decayed roots allow water to infiltrate more rapidly. Infiltration is reduced by 70-80 percent or more in urban areas with buildings and pavement, compared to native woodland and prairie watersheds.

Rainwater or snowmelt that moves over the land rather than entering the ground by infiltration is called runoff. If precipitation or snow melt exceeds the infiltration rate, water will accumulate on the surface and run off. Steep slopes encourage runoff and limit infiltration. In areas where rainfall occurs at very high rates, groundwater increases may not occur since the ground has little time to absorb the water into the subsoil and groundwater. While there may be more flooding occurring on the Earth’s surface because of high volume rain events, the water table in those areas may remain low because of storm water flow away from the area.Infiltration varies by soil type, organic matter content, vegetative cover, and land use  Please refer to Table 1.2)  for a comparison of runoff rates for different surfaces and soils.

Bioenergy crops that will provide for greater infiltration include poplar and willow, especially if they are in mixed stands. Miscanthus, switchgrass and alfalfa as perennial crops will behave more like pasture crops in restricting runoff and encouraging infiltration. Corn, cotton, soybeans, mustard family members (canola, mustard, oil seed radish, etc.), sugar cane and other row crops will provide for less infiltration, particularly prior to canopy closure, and greater potential runoff during crop establishment and after harvest.

Table 1.2.  Relative infiltration and runoff rates of different soils and sites to other soils and sites (10) .

Resources by eXtension, Bioenergy & Sustainability, BIOEN3 CC BY-NC-SA 2.5
www.extension.org/pages/63365/bioenergy-curriculum

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