Cotton is an important economic crop and ranks first among economic crops in planting area in China. There are five cotton areas which are located at South China, North China, Yangtze River basin, Yellow River basin and North-West arid regions of China. The water requirement of the whole growth stage of cotton is different in different area, which is specified as 400mm to 1000mm or more. Drought is the key factor that hinders the yield and quality of cotton in the North and North-West of China. Also the frequency of drought in different growth stages is high in some regions because of the temporal mismatch between precipitation and water-consumption of cotton and local people have to irrigate in arid years. It is believed that conventional irrigation is a luxury use of water and can be reduced without much effect on economic yield. Methods that may cut down irrigation are of considerable interest and should be explored.
To optimize irrigation and provide a technical guide on agricultural production,the water-saving irrigation meteorological grading indexes in different growth stages of cotton are built in this paper, based on the farmland water balance principle, the theory of insufficient irrigation and regulated deficit irrigation. Multidimensional information including daily meteorological data from 347 meteorological stations from 1961 to 2008, soil moisture data every ten-day from 89 agro-meteorological stations from 1993 to 2008 (in which 26 agro-meteorological stations from 1980 to 2008),irrigation amount and yield data are collected to analyze the relationship between the moisture deficiency rates, the quantity of water deficit and yield reduction rates of sowing -jointing stage, jointing -heading, heading-mature period and the whole growth stage of cotton referring to the product-water function (FAO-1979). Results indicate that water-saving irrigation meteorological grading indexes fromⅠ-Ⅲ level of moisture deficiency rate(D)are respectively for 15%≤D < 30%, 30%≤D < 45% and ≥45%, corresponding to 1-2 irrigations(600m³/hm² each time), 3-4 irrigations and ≥4 irrigations for the quantity of water deficit (secondary indicators) for the whole growth stage of cotton. For sowing-budding stage, the indicators are respectively for 20%≤D < 35%, 35%≤D < 50% and ≥50%, corresponding to 0.5 irrigation, 1 irrigations and >1 irrigations. For budding-flowering stage, the indicators are respectively for 20%≤ D < 35%, 35%≤ D < 45% and ≥45%, corresponding to 1 irrigation, 1-2 irrigation and >2 irrigation. For flowering-harvesting stage, the indicators are respectively for 20%≤ D < 35%, 35%≤ D < 50% and ≥50%, corresponding to 1 irrigation, 1-2 irrigation and >2 irrigations.
The index system set up in the paper is elaborated for the first time as a drought monitoring index to explore its applicability in different development stages of cotton. Besides that the drought warning and forecasting is developed by considering the weather forecast and water consumption of crops which is calculated by referring the FAO-56 function. the conclusions are based on the current level of production and management, natural water supplies and artificial irrigation. In this paper, we introduce the product-water function as the theoretical basis and use multisource data to prove these conclusions. However, there are some limitations. Firstly, we use the referring value of FAO-56 to represent the crop-coefficient approximately. Secondly, we use the precipitation index to replace the effective precipitation, ignoring the precipitation type. Thirdly, we use trend yield under drought year to express approximately the maximum yield by referring the FAO-1979 function. Actual irrigation capacity and detailed indexes should be considered in further studies.