Page 123 - Biosystems Engineering

104 Cha pte r T h ree

†
R or
h
Method T * e ‡ Latitude Elevation R § u ¶
a s
Penman (1948) ▲ ▲ ▲ ▲ ▲
Jensen-Haise (1963) ▲ ▲ ▲
Blaney-Criddle (1950) ▲ ▲

Thornthwaite (1948) ▲

∗ Air temperature; relative humidity; actual vapor pressure of the air; solar radiation;
‡
†
§
¶ wind speed.
TABLE 3.1 Minimum Climatic Data Needs of Some Common ET Estimation
Methods (Adapted from Ward and Trimble 2003.)

computed by multiplying ET with a crop coefficient. Crop coefficients
p
are developed by statistical analysis of observed data.
There are various methods for estimating ET. What method to use
usually depends on the available weather data for the site of interest.
Table 3.1 lists some of the available methods and the minimum cli-
matic data needs. ET is computed for a reference crop, which is usu-
ally either grass or alfalfa. This computed ET is called reference crop ET.
Then, crop coefficients that were developed for the study area are used
to compute the actual ET for the desired crop. Detailed description of
ET estimation methods listed in Table 3.1 is rather lengthy. We refer
interested users to Jensen et al. (1990) and Ward and Trimble (2003).

3.5 Infiltration and Runoff
Infiltration and runoff are two essential parts of the hydrologic cycle.
A portion of rain reaching the soil (i.e., rainfall minus interception
losses) is partitioned into two parts. Depending on rainfall intensity,
and soil characteristics and conditions, some part infiltrates and the
excess part that does not infiltrate runs off. Infiltrated water supplies
the water needed by plants and replenishes the soil moisture. Some of
the infiltrated water may further percolate and recharge the ground-
water. Infiltrated water may also move horizontally in shallow soils
and produce subsurface stormflows. Surface runoff may discharge into
creeks, streams, rivers, lakes, reservoirs, or any other waterbodies.
Both very high and very low runoff could have undesirable conse-
quences, such as drought and flooding. Subsurface stormflows could
also have adverse impacts on water quality. Low-flow conditions may
alleviate pollutant concentrations. Similarly, high runoff rates acceler-
ate erosion and cause soil loss and higher turbidities in waterbodies.