The Texas High Plains represents the largest contiguous cotton production region in the U.S. The planted acreage within a 150 km (100 mi) radius of Lubbock, TX approaches 1.5 million hectares (4 x 10 ac) annually. Three major environmental constraints exist which limit yield. The single most important environmental limitation is lack of an adequate water supply throughout the growing season. Precipitation averages 450 mm (18 in) annually with most of the rainfall occurring during the growing season (April-September). However, potential evaporation approaches 10 mm (0.4 in) per day during the growing season resulting in considerable water stress especially during July and August. About one-half the area has access to supplemental irrigation. Under irrigated conditions, the major environmental constraint becomes the thermal energy supply. Average annual heat unit (base temperature = 15 C) accumulation ranges from 1200 (2200 DD-60's) on the northern edge to 1400 (2500 DD60's) on the southern edge. Under irrigated conditions, yields are fairly well correlated with seasonal heat unit accumulation with the most critical period being during early reproductive development (40-70 DAE). Under dryland conditions, a negative correlation exists between yield and heat units when the seasonal accumulation exceeds 1000 (1800 DD-60's). Management of the water resource within the confines of the thermal environment becomes a major management consideration. The third major yield constraint is the concentration of N and P in the soil. A recent soil fertility survey conducted by the High Plains Underground Water District revealed that the soil levels of N and P in the root zone (top 1 m, 3 ft) were low to very low (less than 10 ppm N and P) in over 70% of the counties surveyed. The sandy soils in the southern half of the area are especially low in N.

Yields in excess of 1000 kg/ha (2 bales/acre) are attainable essentially every year when proper management is used. Under dryland conditions, the rainfall received during the growing season and stored in the soil profile is adequate to support over 500 kg/ha (1.0 bale/acre) yields almost every year.

Our research program has been concerned with developing a comprehensive management system which minimizes the impact of the environmental limitations and maximizes productivity. Our efforts have concentrated on maximizing use efficiency of the available water resource, both rainfall and irrigation. Plant density and spacing considerations based upon the water holding capacity of the soil and the probability for rain are major concerns under dryland conditions. Irrigation scheduling is of major concern when the water supply can be managed. Maintaining a balanced nutrient supply to support the growth potential provided by the water supply requires consideration of 'how much' and 'when' to apply N and P.

In order to integrate all factors (both controlled and uncontrolled) which influence cotton productivity and make more intelligent changes during the course of the growing season, we have been using GOSSYM. Comprehensive data from each of the major soil types in the area have been developed. Weather stations at each location collect complete weather data on an hourly basis. Complete growth analysis is conducted at regular intervals throughout the growing season. Research variables include plant spacing and population, irrigation scheduling, nitrogen and phosphorous rates and times of appreciation etc. The model is used to simulate growth, development and yield. The actual response is compared to the predicted response in terms of both developmental,and physiological responses. The results of our efforts are summarized in the next four papers.