Conservation and consumptive use of water with sugar cane under irrigation in the south coastal area of Puerto Rico

Abstract

1. Prevailing methods of irrigation in Puerto Rico can be reasonably efficient in the use of water (about 50% retained) if the systems are carefully laid out and if the irrigators are well trained and conscientious. With the standard, short-run, big-furrow, McLane methods the greatest losses are caused by applying too much water in one irrigation and by applying water at times when the soil has very little available storage capacity. At its best, the short furrow (McLane) method has a high labor requirement and is therefore rather expensive. Properly designed sprinkler irrigation has shown a consistently high efficiency of about 75%. Major changes in irrigation methods, other than by sprinkling, would require alteration of field lay-outs, land preparation, cultural operations, and labor practices. Further study is needed to determine whether some such alterations might be feasible, and compatible with high cane yields. There are too many interdependent factors to permit much change in irrigation methods without upsetting other features of the system of cane culture as a whole. Details of irrigation methods (9) and of their efficiency (8) have already been reported elsewhere. 2. Regardless of the irrigation methods used, the periods of greatest opportunity for saving water with sugar cane are the first few and the last few months of the crop season. The greatest danger of damage to the crop because of lack of water normally comes during the season of peak growth which also corresponds with the highest average temperatures. Consumptive use of water at this time averages about 0.18 inch per day compared to 0.10 or 0.12 during the first and the last part of the season. 3. Soil moisture guides appear to offer the most promising present basis for determining when to irrigate. By depending upon soil moisture rather than upon arbitrary schedules or field men's judgment it appears to be possible to increase cane yields, save water, and save labor, all at the same time. These indications are being given extensive field scale tests by Luce and Co. at Aguirre. Both mercury type, tensiometers (constructed by the BPISAE shop at Beltsville) and Boyoucos type nylon resistance blocks are giving satisfactory results. The blocks are preferred because of simplicity of operations. Normal salt variations in soil have not affected block readings. Inherent block errors and block failures have been satisfactorily overcome by using 4 or more replicates at carefully selected stations representing a unit irrigation area. Any blocks which deviate seriously from the average are removed and replaced. The resistance or tension readings which serve as the basis for irrigation have been established by our tank and field studies and by laboratory soil moisture tension curves. For soil like the Santa Isabel clay in the area from Juana Díaz to Aguirre it is not safe to let the soil moisture tension in the main root zone of cane go much beyond one atmosphere. With any Puerto Rican soil a safe tension for irrigation should probably correspond with a point which is at least 5% above the wilting point on a laboratory pF (moisture retention) curve. 4. Present field results indicate that high sugar cane yields per acre probably mean less water use per unit of crop produced. This is the basis for a field scale experiment by Luce and Co. comparing two, block-controlled soil moisture levels, each with two levels of fertilization. 5. Under Puerto Rican conditions, crop characteristics and soil moisture levels probably overshadow the influence of variations in the weather factor on evapotranspiration much more often than under climates of the temperate zone where the weather factor is highly variable. In any detailed considerations of climatic influences, the weather records from Aguirre, San Juan, and Mayagüez, indicate that differences in wind movement should be given major consideration along with hours of sunshine and seasonal temperatures.