Soil moisture Variations in Upland Field

Abstract

Thinking of the mechanism of water decrease in upland, the most important problem is thought to be the prediction of soil moisture content in fields by the indirect element such as meteorological factors. In this case, the soil properties and soil conditions must be cleared up as the preceding conditions first of all. Above all, it is necessary to set up the range of available soil moisture for crop productivity.The initial water content in the soil layer was disccused in detail how to be decided under the natural conditions. The author put next a new definition on the field capacity, that is, “Field capacity is the water content at the time when the amount of drainage water in the soil layer just under the available root zone decrease remarkably curing the night after suppling much water or having much precipitation over the holding capacity of soil layer.” In southern Kyushu, we can see these conditions about 48 hours after precipitation for volcanic ash soil. And then, the water content at 48 hours (W48) become nearly constant value in the indication of the degree of water saturation among the soil layers with different porosities. If we use these values, we can find the initial water content of the soil layer by knowing the amount of porosity. For instance, if we take the soil layer 60cm depth, the amount of porosity ∑Po60, and the degree of water saturation SA60 then the initial water content in the 60cm soil layer (WI60) will be expressed as follows, WI60=SA60×∑Po60=0.80×∑Po60(SA60=0.80……observed value)Next, if we analyze the water consumption with the water balance method, we must divide the moisture flux into the upward flow and the downward flow. As to the evapotranspiration, the downward moisture flow must be excepted from the calculation. What is the critical value dividing the flow into the upward or the downward? The author considered of this problem from the standpoint of the environmental evaporation capacity, which is calculated as the latent heat flux in the heat balance equation. Judging by the comparison, the critical value is thought to be the value of pF1.6, and that corresponds to the 48 hours water content (≅field capacity) for volcanic ash sandy loam in southern Kyushu.We examined the relationships among the estimated amount of evapotranspiration from the heat balance method (lEo/l), those from the net radiation (S/l), tne' observed evaporation with 20cm pan evaporimeter (Ew) and the observed evapotranspiration from the soil layer of 0-50cm by the water balance method (pET). As the results of these comparisons, the relationships, pET=lEo/l=EW……(W≥W.c.)were found out in our experimental fields throughout the duration from July to November.The water content in the soil layer of 50cm (W50) is obtained from, W50=SA×∑Po50-WW50-pET·t……(W≥W.c.)W50=(SA×∑Po50-WW50)·exp(-W/W.c.×pET·t)…(W<W.c.)where, t: elapsed days from the 2 days after rain or irrigation, W: available water content, WW: unavailable water content, nearly equall to the wilting point, PET: potential evapotranspiration, ∑Po: porosity.When it is little rain on the way of drying process, we can compensate the soil moisture content adding the amount of rain to the right side of the above mentioned equations. Besides, for indirect prediction, we had better use the accumulated amount of observed values day by day such as