Yoshida - 1981 - Fundamentals of Rice Crop Science

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CLIMATIC ENVIRONMENT AND ITS INFLUENCE 105 2.20a. The water balance of a root zone for lowland rice. b. Some major relationships between the hydrologic cycle, waters on the land, and the air-mass cycle (from An Introduction to Climate by G. T. Trewartha 1968. Copyright © 1968 by McGraw-Hill, Inc. Used with the permission of McGraw-Hill Book Company). Under most conditions, W in = P + I, (2.34) where P is rainfall and I, irrigation. On sloping topography, surface runoff or
106 FUNDAMENTALS OF RICE CROP SCIENCE overland flow from fields at a higher level can also be taken as a component of W in . At the same time, water may be withdrawn from the soil ( W out ) by evapotranspiration ( E ), percolation and seepage ( P & S ), and runoff or overland flow. Under the simplest situation and disregarding runoff or overland flow, the water balance can be formulated as: D W = P + I – E – P & S. (2.35) This equation states the law of mass conservation with respect to water. To compute D W, however, the rooting depth of a crop and the soil moisture characteristics in the field must be known. Unfortunately, very little is known of the rooting depth of rice in the field. To estimate water requirements or the irrigation interval, however, knowledge of the water storage capacity of the soil is not necessary. If the initial moisture conditions of the soil are specified, the water requirement or irrigation need can be obtained from: D W = O or P + I = E + P & S. (2.36) The initial condition could be the water depth set up for irrigated lowland rice, or the field moisture capacity for upland crops. In equation 2.36, ( P + I ) is the amount of water required to maintain the initial condition. E can be estimated by various methods as discussed in the preceding sections and ( P & S ) can be measured directly or its estimated value can be obtained for representative soils. For example, the average percolation and seepage rates for paddy soils in the Philippines are estimated at 2 mm/day for the wet season and 4 mm/day for the dry (Wickam and Singh 1978). Thus, the total monthly water requirements of a rice crop in the Philippines average about 180 mm for the wet season and about 270 mm for the dry (Table 2.15). These estimates agree with experimental results on irrigation needs. Monthly water requirements for rice are about 200 mm. The average total requirement for 1 rice crop is about 1,200 mm, assuming that the crop duration is 5 months and that land preparation requires 200 mm water. Under rainfed conditions and disregarding ( P & S ), the equation 2.35 can be reduced to: D W = P – E (2.37) The equation 2.37 may be regarded as a climatic water balance; it gives a rough estimate of water shortages or excesses (Fig. 2.21). Wet season is the time when rainfall exceeds evapotranspiration. At Los Baños, Philippines, January to April is the dry season and June to December is the wet season. May is a transition period. The wet season thus defined agrees well with the crop period for the rainy-season rice crop. Water deficit or drought occurs at unpredictable times. From 1967 to 1971, a monthly water deficit occurred three times during the wet season — July 1967,
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FOREWORD Rice is vital to more than
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CONTENTS Foreword Preface GROWTH AN
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3.2. Adaptation to Submerged Soils
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4.5. Corrective Measures for Nutrit
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1 .1. OUTLINE OF THE LIFE HISTORY T
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GROWTH AND DEVELOPMENT OF THE RICE
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Page 63 and 64: GROWTH AND DEVELOPMENT OF THE RICE
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5.1. PHOTOSYNTHESIS 5.1.1. Photosyn
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PHOTOSYNTHESIS AND RESPIRATION 197
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Photosynthetic characteristics Meas
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A traditional tall variety vs an im
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Table 7.6. An example of high yield
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REFERENCES AGRICULTURAL POLICY STUD
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Yoshida - 1981 - Fundamentals of Rice Crop Science

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