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Concurrent Evaluation of Agronomic, Economic, and Environmental Aspects of Trickle-Irrigated Watermelon Production

Dep. of Biological Systems Engineering, Univ. of Nebraska-Lincoln, 233 LW Chase Hall, Lincoln, NE 68583-0726;

T. A. Doerge^*

Dep. of Soil and Water Science, Univ. of Arizona, 429 Shantz Bldg. 38, Tucson, AZ 85721.

^* Corresponding author.

ABSTRACT

Reducing application rates of water and N fertilizers in irrigated cropping systems can lower the potential for N losses, but increases economic risk to producers. A data normalization method and an abstract spatial analysis procedure examined yield, net economic return and unaccounted for N from a subsurface, trickle-irrigated watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai] cropping system in southern Arizona as a function of N and water inputs. Field research was conducted on a Casa Grande sandy loam (fine-loamy, mixed, hyperthermic Typic Natrargid). A factorial design consisting of four levels of N (60, 216, 315, and 500 kg N ha^–1) and three average soil water tensions (4, 7, and 17 kPa) resulted in a yield response surface. Watermelon marketable value and costs of water and N inputs were estimated to determine a net return response surface. A N mass balance was calculated by the difference method to estimate post harvest unaccounted for fertilizer N. Predicted maximum marketable yield was 102 Mg ha^–1 at 7.2 kPa tension and 336 kg N ha^–1. Predicted maximum net return was $10 819 ha^–1 at 10.2 kPa tension and 256 kg applied N ha^–1. Predicted maximum unaccounted for N was 300 kg N ha^–1 at 4 kPa tension and 500 kg applied N ha^–1. Normalization and summation of yield, net return and unaccounted for N response variables resulted in a predicted optimum response at 12.6 kPa and 178 kg N ha^–1. The combined response variable was within 95% of this maximum value across the range of 10 to 16 kPa tension and 60 to 300 kg applied N ha^–1. Similarly, spatial analysis of the three response variables indicated 7 to 17 kPa tension and 60 to 315 kg applied N ha^–1 would result in yield and net return of >95% of the calculated maxima of marketable yield and net return, while limiting calculated NO^–₃-N concentration in soil water draining below the root zone to <10 mg NO^–₃-N L^–1. These results suggest that data normalization and abstract spatial analysis are useful in concurrent evaluation of agronomic, economic, and environmental production criteria for subsurface trickle-irrigated watermelon.

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