Wiedenfeld, R.P. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 SUGARCANE WATER USE AND IRRIGATION REQUIREMENTS IN A SEMI-ARID ENVIRONMENT

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1 SUGARCANE WATER USE AND IRRIGATION REQUIREMENTS IN A SEMI-ARID ENVIRONMENT By R.P. WIEDENFELD Texas Agricultural Experiment Station b-wiedenfeld@tamu.edu KEYWORDS: Evapotranspiration, Crop Coefficients, Sub-surface Drip Irrigation, Water Use Efficiency. Abstract WATER for agricultural irrigation is becoming increasingly limited and, therefore, must be used as efficiently as possible. Sugarcane is a high biomass crop requiring lots of water. A field study was conducted to determine water requirements and crop water use by applying different levels of water. These were achieved by varying crop coefficients used with reference evapotranspiration to determine irrigation scheduling. Sugarcane yield responses to different water levels varied annually. The irrigation level producing maximum yields was not the same each year. Total water input including rainfall and irrigation to achieve maximum yields varied from 973 to 1328 mm/y; while water use efficiency (fresh weight of cane produced per unit of water uptake) varied from 6.0 to 11.7 t cane/ml of water. These results suggest that the amount of water required to produce maximum yields in the Lower Rio Grande Valley of Texas may be less than the amount theoretically used by this crop under ideal conditions as reflected in the established crop coefficient curves. Deficit irrigation may lower yields, but may increase the amount of cane produced per unit of water used by the crop. Introduction Water availability for agricultural production is becoming increasingly limited everywhere irrigated agriculture is done. Sugarcane is a big water user. This crop is capable of producing 10 t/ha of cane for each 1.0 to 1.2 ML of water used (Holden, 1998; Rozeff, 1998). Based on evapotranspiration rates, sugarcane therefore has the potential to produce up to135 t/ha depending on climate conditions in the Lower Rio Grande Valley, which would require 13.5 to 16.0 ML of water. While many factors including inadequate nutrition, salinity, weeds, insects and diseases can impact sugarcane production, water stress is most often the primary limitation. Sugarcane irrigation in the Lower Rio Grande Valley is usually very inefficient and wasteful. Therefore, substantial improvements are possible (Wiedenfeld et al., 2005). Several efforts have been made to establish sugarcane crop water requirements in South Texas (Salinas and Namken, 1977; Wiedenfeld, 1995). In order to irrigate sugarcane as efficiently and effectively as possible, it is necessary to have a good understanding of crop water requirements and use in a manner that can be easily related to annual variability in climatic and rainfall conditions. The purpose of this study was to identify optimum water application levels based on evapotranspiration for sugarcane growth, yield and quality. Materials and methods A field study was initiated in 2000 in the Lower Rio Grande Valley of Texas (26 o 10 N, 97 o 56 W, elevation 18 m), an area with a subtropical, semi-arid climate. Sugarcane cultivar 351

2 TCP was planted on 18 September 2000 on a Raymondville clay loam soil (ph 8.4). Treatments consisted of three irrigation levels determined by using mid-season crop coefficients (K c mid) of 1.0, 1.25 and 1.5 (Figure 1). Fig. 1 Crop coefficient curves used to calculate crop water use by sugarcane and therefore water applied to the three irrigation treatments. Sugarcane crop water requirements were then determined by applying the appropriate coefficient curve to reference evapotranspiration (ETo) calculated with the Penman-Monteith equation (Allen et al., 1998) using data from an automated weather station. A water balance approach was used, assuming an available soil moisture holding capacity of 0.16 cm water per cm soil depth, and a rooting depth that increased from 61 cm to 152 cm over time during the growing season. The target of the irrigation scheduling was to initiate irrigation when available soil moisture reached 55% depletion. The volume of water applied in the plant crop was the amount necessary to refill the profile while, in the 1 st through 3 rd ratoon crops, the volume applied was the amount necessary to raise available soil moisture content to 25% depletion. During the growing season over the four year period, soil moisture was maintained above 55% depletion 84% of the time. The greatest number of stress days occurred in the 1 st ratoon crop, and the fewest in the plant crop. When soil moisture levels occasionally fell below the minimum depletion level, actual crop water use fell below potential crop water use, as accounted for by the stress coefficient in the Penman-Monteith equation. Also, occasional heavy rainfall events caused water inputs greater than the available storage capacity of the soil profile resulting in runoff or drainage loss, and therefore effective rainfall lower than total rainfall. This occurred most often in the plant crop when irrigation volume applied was the amount necessary to refill the soil profile. Effective rainfall in the plant crop averaged 50.1%, and in the 1 st through 3 rd ratoon crops averaged 72.6%. Daily rainfall events below 2.5 mm were also considered ineffective. Calculated soil moisture depletion levels for the middle irrigation treatment (K c mid =1.25) in the 3 rd ratoon crop over time is shown in Figure 2. (A separate graph for each treatment in each crop year was developed for irrigation scheduling.) 352

3 Fig. 2 Soil moisture depletion over time during the 3 rd ratoon crop (2004) for irrigation treatment K c mid = 1.25 based on calculated crop water use, water inputs and soil water storage capacity. The target range of 25 to 55% depletion is illustrated. Irrigation and rainfall events are also shown.irrigation was stopped each year on 30 September to induce sugar accumulation in the crop. The actual amount of irrigation and rainfall for each treatment each year is shown in Table 1. Table 1 Crop evapotranspiration (ETc), water inputs, cane yield and water use efficiency for the three irrigation treatments for four sugarcane crops. Etc 1 Rain 2 Combined Cane Water use Irrigation Year Crop Days ETo Kc Potential Actual Total Effective water inputs yield 3 efficiency cm cm t/ha t/ml 2001 Plant st Ratoon nd Ratoon a ab B rd Ratoon a B a Actual crop evapotranspiration may be less than potential crop evapotranspiration due to water stress occasionally encountered. 2 Effective rainfall is less than total rainfall due to various losses before the water is used by the crop. 3 Means followed by the same letter are not statistically different. Where no letters follow means, no statistically significant difference was found. 353

4 Wiedenfeld, R.P. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 Treatments were applied in plots 125 m2 in size (13.7 m long by 6 rows at 1.5 m spacing) and were replicated five times in a randomised complete block design. The crop was irrigated using sub-surface drip irrigation using water from the Rio Grande River [average electrical conductivity (EC) 1.3 ds/m). Lateral connections for water delivery were configured to allow control and metering for each individual plot. The study was conducted for four sugarcane crops, which were harvested annually for yield and sugar analysis. The field was burnt then harvested using a commercial harvester and weigh wagon to weigh the entire plot. Subsamples of cane were taken, crushed, and the juice analysed for refractometer solids, pol and EC. Results and discussion Sugarcane crop water use determined using the crop coefficient curve based on the low Kc =1.0 annually ranged from 10.8 to 13.0 ML depending primarily on the length of the growing mid season (Table 1). The increase in crop water use when the high Kc mid = 1.5 was applied was 44% higher in the plant crop, and 26 to 27% in the 1st through 3rd ratoon crops. No significant differences in sugarcane yields occurred due to the irrigation treatments applied in the plant and 1st ratoon crops (Figure 3). Fig. 3 Sugarcane fresh weight yields for the three irrigation treatments for four crop cycles. Highest yields in the 2nd and 3rd ratoon crops occurred at the low and middle irrigation level, respectively. No differences in juice quality parameters due to the irrigation treatments were found any year. Between the highest and lowest irrigation treatments each year, the level of variation in sugarcane yield was 15% or less, while the difference in the amount of water inputs ranged between 23% and 39%. Therefore, highest water use efficiency each year occurred at the lowest irrigation level and declined with increasing water application (Figure 4). The first through third ratoon sugarcane crops in this study produced between 9.3 and 11.7 t of cane per ML of water used at the low irrigation treatment. This is in line with the published production potential for sugarcane (Holden, 1998, Rozeff, 1998), and is well above typical production for this region. 354

5 Fig. 4 Sugarcane water use efficiency (fresh weight of cane produced per unit of water uptake) for the three irrigation treatments for four crop cycles. This suggests that other stress factors were probably not limiting production. The fact that yield gains due to increases in water application were small or nonexistent suggest that the amount of water required by sugarcane in order to produce maximum yields in the Lower Rio Grande Valley of Texas may be lower than the total amount of water used by sugarcane under ideal condition based on the published crop coefficient curves. REFERENCES Allen, R.G., Pereira, L.S., Raes, D. and Smith M. (1998). Crop evapotranspiration guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome. Holden, J.R. ed. (1998). Irrigation of Sugarcane. BSES, Brisbane, Australia. Rozeff, N. (1998). Sugarcane irrigation management. In: Rozeff, N. et al. ed. South Texas Sugarcane Production Handbook. Texas A&M Univ. Res. Ext. Ctr., Weslaco., Salinas, F. and Namken, L.N. (1977). Irrigation scheduling for sugarcane in the Lower Rio Grande Valley, Texas. Proc. Am. Soc. Sugar Cane Technol., 6: Wiedenfeld, R.P. (1995). Effects of irrigation and N fertiliser application on sugarcane yield and quality. Field Crops Res., 3: Wiedenfeld, B., Enciso, J., Fipps, G. and Robinson, J. (2005). Irrigation of sugarcane in Texas. Texas Cooperative Extension B-6156 (15 p). 355

6 UTILISATION DE L EAU ET BESOINS EN IRRICATION DE LA CANNE À SUCRE DANS UN ENVIRONNEMENT SEMI-ARIDE Par R.P. WIEDENFELD Texas Agricultural Experiment Station b-wiedenfeld@tamu.edu MOTS-CLÉS: Evapotranspiration, Coefficients Culturaux, Irrigation Goutte a Goutte, Efficience d'utilisation de l'eau. Résumé L'EAU NECESSAIRE à l'irrigation agricole, devenant de plus en plus limitée, doit être employée le plus efficacement possible. La canne à sucre, dont la production de biomasse est élevée, exige de grandes quantités d'eau. Une étude au champ a été conduite pour déterminer les besoins en eau et l'utilisation de l'eau par la culture en appliquant différentes doses d irrigation. Ceci a été réalisé en faisant varier les coefficients culturaux employés avec l'évapotranspiration de référence pour déterminer les programmes d'irrigation. Les réponses du rendement de la canne à sucre aux différents apports d'eau ont varié selon les années. Les apports produisant les rendements maxima diffèrent chaque année. L'apport total d eau incluant les précipitations et l'irrigation, pour réaliser les rendements maximum, a varié de 973 à 1328 mm/an; tandis que l efficience d'utilisation de l'eau (rendement frais/quantité d eau absorbée) a varié de 6.0 à 11.7 T canne/ml d eau. Ces résultats suggèrent que la quantité de l'eau nécessaire pour produire les rendements maxima dans la basse vallée du Rio Grande du Texas peut être inférieure à la quantité théoriquement employée par cette culture dans des conditions idéales indiquées dans les courbes de coefficients culturaux. Une irrigation déficitaire peut diminuer les rendements, mais peut augmenter la quantité de canne produite par unité d eau utilisée par la récolte. USO DE AGUA EN CAÑA DE AZÚCAR Y REQUERIMIENTOS DE RIEGO EN UN AMBIENTE SEMI-ÁRIDO Por R.P. WIEDENFELD Texas Agricultural Experiment Station b-wiedenfeld@tamu.edu PALABRAS CLAVE: Evapotranspiración, Coeficientes de Cultivo, Riego por Goteo Sub-Superficial, Eficiencia de Uso del Agua. Resumen EL AGUA para riego agrícola se está volviendo más limitada y por tanto, debe ser usada lo más eficientemente posible. La caña de azúcar es un cultivo de alta biomasas que requiere bastante agua. Se condujo un estudio de campo para determinar los requerimientos hídricos y el uso de agua del cultivo aplicando diferentes niveles de agua. Esto se logró variando los coeficientes de cultivo usados con la evapotranspiración de referencia para determinar los calendarios de riego. Las respuestas en el rendimiento de la caña de azúcar a los diferentes niveles de agua variaron anualmente. El nivel de riego que produjo los máximos rendimientos no fue el mismo cada año. La cantidad de agua entrante incluyendo lluvia y riego para alcanzar los máximos rendimientos varió de 973 a 1328 mm/año; mientras que la eficiencia de uso del agua (peso fresco de caña producida por unidad de agua tomada) varió de 6.0 a 11.7 t caña/ml de agua. Estos resultados sugieren que la cantidad de agua requerida para producir máximos rendimientos en el Bajo del Valle del Río Grande en Texas puede ser menor que la cantidad teóricamente usada por este cultivo bajo condiciones ideales tal como se reflejó en las curvas determinadas de coeficiente de cultivo. Un riego con déficit podría bajar los rendimientos, pero podría aumentar la cantidad de caña producida por unidad de agua empleada por el cultivo. 356