Spatial variation of sugarcane water requirement and irrigation quota in Guangxi

Abstract: Guangxi Province, the major sugarcane production base in China, is of significant importance in ensuring the national sugar industry security duo to its high and stable sugarcane output. In order to understand the laws of water requirement of Guangxi sugarcane, guide sugarcane irrigation in different parts of Guangxi, achieve water conservancy modernization and consolidate the national base of sugarcane production, contour map of water requirement and irrigation norm of sugarcane covering the entire area of Guangxi were drawn and their associated spatial variability was investigated. A total of 3-year sugarcane irrigation tests were carried out in 7 experimental stations located in North Guangxi (Luzhai and Liuzhou stations), Central Guangxi (Wuxuan, Wuming, Fusui and Jiangzhou stations) and South Guangxi (Hepu station) from 2012 to 2014. The difference of sugarcane yield under the condition of various irrigation methods and norm was studied and data base of soil and irrigation were simultaneously established. Additionally, basic meteorological data (daily highest temperature, lowest temperature, relative humidity, etc) were collected by 25 nationally meteorological stations in this region. Based on existing data of soil and growth period of sugarcane from the 7 experimental stations, planting area of sugarcane in Guangxi was divided into North, Central, West and South Guangxi. In the aforementioned 7 experimental stations, Luzhai, Wuxuan, Jiangzhou, Fusui, Wuming and Hepu represented North Guangxi, east of Central Guangxi, west of Central Guangxi, West Guangxi and South Guangxi, respectively. According to this division and the meteorological data of nearly 60 years from the 25 meteorological stations, crop reference evapotranspiration ET0 were estimated based on Penman-Monteith formula. Sugarcane crop water requirements ETc were estimated through dual crop coefficient method under 4 different irrigation methods of drip irrigation, small tube flow irrigation, micro-spray irrigation and furrow irrigation. And irrigation quota was obtained on the basis of water balance equation of crop in arid area. The corresponding contour maps were obtained according to the tension spline interpolation. Relative error between calculated and experimental data was from -1.9% to 16.7%, according to the comparison with ETc of Luzhai station and Jiangzhou station, where relatively complete data were collected during 2012 to 2013. The smaller error indicated that the dual crop coefficient method proposed here was reasonable. For convenience, Guilin of North Guangxi, Nanning of Central Guangxi, Baise of West Guangxi and Beihai of South Guangxi was considered as the representative sites, and their variation coefficients of daily average ET0 during the growth period of sugarcane were 5.1%, 5.7%, 4.5% and 4.1%, respectively. For drip irrigation, the variation coefficient of daily average ETc were 6.4%, 7.2%, 5.3% and 5.2%, respectively, the irrigation quota were 25.9%，22.8%，21.8% and 21.9%, and the effective rainfall were 17.6%, 18.0%，19.0% and 20.7%, respectively. The results showed that the annual daily average temperature presented downward trend from low to high latitude among the growth period of sugarcane. The maximum point of daily average effective rainfall was located in Dongxing region. Difference of effective rainfall across Guangxi reached 65%, indicating the uneven spatial distribution of rainfall. The area from Baise to Tiandong was the district with small rainfall. Baise was the lowest point of effective rainfall in Guangxi with 1.94 mm/d annual daily average effective rainfall, which only accounted for 36% of Dongxing. However, the average temperature between Baise and Dongxing were similar among the whole growth period of sugarcane, which were 24.04 and 24.88℃, respectively. Peak point of daily average ET0 was located in Beihai, which equaled to 3.88 mm/d. Tiandong was the regional peak point of West Guangxi, and the responding value was 3.67 mm/d. The results showed that spatial distribution of ETc under the different irrigation methods presented almost the same distribution law, which was gradually reduced from southeast to northwest. Beihai was the peak point across the whole Guangxi, and Tiandong was the local peak. Difference in spatial distribution of daily average ETc under the 4 irrigation methods were all about 40%. The value of ETc was 3.48, 3.48, 3.59 and 3.52 mm/d to drip, small tube flow, micro-spray and furrow irrigation, respectively. Irrigation quota under different irrigation methods also presented almost the same spatial distribution. Area with high value was the region from Guilin to Tiandong, the value of the region from Guiping to Lingshan was lower than the region of Tiandong. The region from Dongxing to Fangcheng was located in the low area of irrigation quota, where the value was just a little higher than the lowest one. The peak of irrigation norm across Guangxi was located in Tiandong, followed by Guilin and Beihai. Irrigation norm under the 4 methods of drip, small tube flow, micro-spray and furrow irrigation were 135, 135, 354 and 457 mm. Soil wetting proportion of drip, small flow tube, micro-spray and furrow irrigation were 0.35, 0.35, 0.85 and 1.00, respectively, and the irrigation quota increased with the increase of soil wetting proportion. Irrigation quota under micro-spray irrigation mode increased 147% compared with that under drip and small flow tube irrigation. The increase of irrigation quota under furrow irrigation mode was 2 times larger (232%) than that under drip and small flow tube irrigation, and lightly higher (34%) than micro-spray irrigation. Spatial heterogeneity of meteorological data, especially the rainfall data, led to the dramatically difference in irrigation space distribution of Guangxi sugarcane. Results could provide valuable information for irrigation application in sugarcane.