Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Biosystems Engineering

Advisor

Khalilian, Ahmad

Committee Member

Privette , Charles V

Abstract

Cotton (Gossypium hirsutum L) is cultivated in many countries under both rainfed and irrigated conditions. In the U.S.A., cotton is grown in 17 states across a vast region known as the Cotton Belt. In the Cotton Belt as with many parts of the world, irrigated cotton is a considerable water user, but for good reasons. Irrigation can boost yield as well as stabilize yield and quality by ensuring adequate soil water during the entire growing season or at least during critical growth stages in areas where water resources are limited. From 2002 to 2007, irrigated acreage in the western states declined significantly, while in the southeastern states, irrigated acreage increased by 70%. Recent drought periods (1998-2002, 2007, & 2011) in the southeastern U.S.A. and trans-boundary water conflicts between neighboring states have elevated the importance of water resources conservation. Competition for limited water resources has become a critical issue in some parts of the southeastern states. For example, in some parts of Georgia, limits are already being placed on agricultural irrigation. In this environment, the challenge for the coming decades will be increasing food and fiber production with less water. This can be partially achieved by increasing crop water use efficiency (WUE) - the amount of yield produced per unit water used.
Increasing crop water use efficiency (WUE) and use of more drought tolerance cotton varieties would help to conserve water. Water productivity (WP) provides another way to evaluate efficiency of crop water use. It is defined in this study as 'the aboveground dry matter (kg) produced per unit land area (ha) per unit of water transpired (m)'. However, there are no published values for WP for cotton in the humid southeast.
The first objective of this study was to determine WUE and WP of cotton in this area. Irrigation experiments were conducted in 2009 and 2010 under field environments and in 2011 under a controlled environment utilizing a rainout shelter at the Edisto Research and Education Center, near Blackville, SC. WUE ranged from 0.39 kg seed cotton/m3 water applied to 0.87 seed cotton/m3 water applied (ie., irrigation plus rainfall). WP values were 12.9, 12, and 12.7 g/m2 in year 2009, 2010, and 2011, respectively.
Field experiments could be lengthy and expensive. Crop models have been used extensively to provide an alternative way of pre-evaluating field experiments. Recently, the Food and Agriculture Organization (FAO) of the United Nations developed the AquaCrop, a yield response to water stress model. AquaCrop has been recently tested for various crops under various climates, except in the humid regions.
The second objective of this study was to parameterize and validate the AquaCrop model for cotton under humid climate of the southeast U.S.A. The model was parameterized and validated using quality local datasets as collected under Objective 1 above. Few parameters in Aquacrop were adjusted, such as canopy growth coefficient (CGC), canopy decline coefficient (CDC), water depletion thresholds (p factors), water productivity (WP), and reference harvest index (HIo) to produce field results from the detailed open-field and shelter studies in 2009 and 2011. Model validation involved using the parameterized model to simulate other field experiments in 2010 and comparing results. Results correlated well with simulated values with high correlation coefficients. For example, simulated values for 100% irrigation treatment in 2010 at E5 field in terms of canopy cover (measured by digital camera), soil water content, and cumulative ET were correlated with measured values with R2 of 0.8394, 0.7877, and 0.9918, respectively. A tested AquaCrop provides the necessary tool to study irrigation optimization under varying timing and intensity of drought stress as it may occur during the growing season and due to climate change and variability.

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