Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 97
2.5.4. Water losses: evapotranspiration and percolation plus seepage
In lowland rice fields, water losses largely result from transpiration, evaporation,
and percolation plus seepage. Transpiration is the loss of water through plant
surfaces and evaporation is the loss of water from free water surfaces. The
combined water losses through transpiration and evaporation are referred to as
evapotranspiration.
Transpiration losses increase with leaf area index (LAI) and reach a plateau at a
LAI of 3.5–4.0. At this plateau, the transpiration loss on sunny days is about
6 ± 2 mm/day (Kato et al 1965a), and that accounts for about 90% of the
evapotranspiration losses (Kato et a1 1965b). Evaporation is high at early growth
stages, when the LAI is small, and accounts for most of the evapotranspiration
losses. As LAI increases, evaporation decreases and transpiration accounts for
most of the evapotranspiration losses.
Evapotranspiration is related to meteorological factors and will be discussed in
detail in the succeeding section. Percolation, seepage, and surface run-off, largely
controlled by edaphic factors such as topography and soil characteristics, are
highly location specific. In sloping areas, water may move by gravity from a high
to a low level. In upland fields, evapotranspiration, percolation, and surface
run-off are the major forms of water loss.
Percolation occurs in a vertical direction. Water is lost through seepage by its
horizontal movement through a levee. In practice, percolation and seepage are
combined and taken as a measure of the water-retaining capacity of a field.
Percolation is largely affected by topography, soil characteristics, and the depth
of the water table. In extreme cases, percolation plus seepage ranges from almost
nil on heavy soils to greater than 100 mm on sandy soils. Rice grown on sandy soils
in Japan requires, on the average, about three times more water than rice grown on
clay soils (Fukuda and Tsutsui 1968). Because of topography and soil characteristics,
the percolation-plus-seepage loss is relatively high in Japan and rather low in
many other rice-growing countries. For example, a percolation-plus-seepage loss
of 20 mm/day is considered desirable for productive rice fields in Japan, but
considered exceptionally high in other countries.
The surface water-storage capacity is determined by the levee’s height in
lowland fields and by the slope and roughness of the soil surface in upland fields.
Surface runoff or overland flow occurs when rainfall intensity exceeds the surface
storage capacity and the percolation-plus-seepage rate or infiltration rate. In
lowland fields, the effective daily rainfall is defined as being not less than 5 mm
and not more than 50 mm (Fukuda and Tsutsui 1968). In upland fields, however,
the upper limit of effective daily rainfall should be much less because the loss due
to surface runoff would be greater.
2.5.5. Evapotranspiration
AS explained before, the term evapotranspiration includes evaporation from both
water and plant surfaces. The phase change implied by the evaporation is physically
identical wherever it takes place.