Crop yield response to water - Cra

historically grown under shifting cultivation with long fallow periods (more than 15 years). Atthe turn of the twentieth century, most of Asia’s upland rice areas have made the transition topermanent systems where rice is grown every year, while 14 percent of the Asian upland ricearea still practises shifting cultivation with shorter fallow periods (3-5 years). In Central andWest Africa, the rice belt of Africa, upland areas represent about 40 percent of the area underrice cultivation but involve about 70 percent of the region’s rice farmers.Growth and developmentLowland rice is usually grown in ‘puddled’ fields. Puddling consists of harrowing or rototillingunder shallow submerged conditions, and is done to control weeds, to reduce soil permeability,and to ease transplanting. After puddling, land is levelled under wet conditions. A typicalvertical cross-section through a puddled rice field shows a layer of 0-0.10 m ponded water,a puddled, muddy topsoil of 0.10-0.20 m, a plow pan that is formed by decades or centuriesof puddling, and an undisturbed subsoil. Rice roots are usually contained within the puddledlayer and are quite shallow.The dominant method of crop establishment in most rice areas is by transplanting. Rice is firstraised in a separate seedbed. Seeds are pre-germinated and are broadcast into either a floodedor wet soil surface in the nursery at a rate of 500 to 800 kg/ha. At transplanting (12-25 days afterestablishment for modern cultivars, whereas it takes 40 days or more for traditional cultivars),the plant densities are equivalent to seeding rates of 40-50 kg/ha. Two to three seedlings arenormally hand transplanted in hills 0.15 to 0.30 m apart. Hybrid rice is usually transplanted asone seedling per hill, while some local cultivars are planted with 5-6 seedlings per hill. Each plantdevelops three to seven tillers, dependent on nutrient status, cultivar, and seedling density. In agood crop stand, the number of grain-bearing panicles will reach 400-600/ m 2 in the dry season,and 300-400/m 2 in the wet season (for common indica cultivars in the tropics).Rice can also be established directly in the field by wet or dry seeding (broadcasting pregerminatedseeds onto wet or flooded soil or sowing dry seeds in dry or moist soil). Seeds arebroadcast or sown in rows of 20-30 cm spacing, at the rate of 80 to 250 kg/ha (Rice KnowledgeBank, IRRI). In recent years the trend in Asia is toward more direct seeding. In commercialproduction fields of the United States and Spain, seeds are commonly broadcast from airplanes.The duration of the growing cycle (from germination to maturity) of rice depends on cultivarand location, ranging from 90 days for short-duration modern tropical cultivars, to 180 daysfor traditional or modern cultivars in subtropical and temperate environments. Cultivardifferences in growth duration are determined by changes in the time from germination tobeginning flowering (60 to 150 days), while the time from beginning flowering to maturityis pretty constant and lasts about 30 days in the tropics but can go up to 65 days in cool,temperate regions The growth and development of rice is temperature dependent (Kropff etal., 1994), stopping at average temperatures below the base temperature, which is tentativelyset at 8 °C for rice in AquaCrop. Most tropical cultivars die when, in the early vegetativegrowth phase, the average daytime temperature drops below 12 °C for more than threeconsecutive days. Generally, damage to the pollen occurs when the temperature at floweringis outside the range of 8 to 35 °C. Though many traditional rice cultivars are photoperiodsensitive (shortening day lengths induce flowering), most modern high-yielding ones are not.(See Figure 3 for typical development).106crop yield response to water