Rain water deficit and irrigation demand of major row crops in the Mississippi Delta
The Mississippi Delta is an important agricultural area, producing 67% of the soybean, corn, and cotton grown
in the state of Mississippi. Because irrigation can stabilize and increase crop yields and economic returns, approximately
80% of the water withdrawn from the alluvial aquifer is used for irrigation. This region is experiencing severe declines in
groundwater level, which threaten the sustainability of irrigated agriculture. Therefore, better irrigation management practices
must be developed, but there is lack of knowledge regarding crop water requirements, rain water deficits, and irrigation
demand to improve rain and irrigation water use efficiencies of these crops. The objective of this study was to determine
the crop water requirements, rain water deficits, and irrigation demand of the three crops using the RZWQM2 model based
on a 100-year representative weather data series from the Stoneville weather station for improving irrigation management
in the Mississippi Delta. The analysis indicated that mean monthly precipitation in the crop non-growing season from November
to the following April was 127 mm, while only 82 mm on average was received during the crop growing season.
Soybean, corn, and cotton are typically planted in early May, late March, and late May and harvested in mid-September, late
August, and late October, respectively. Rainfall during the growing seasons for soybean, corn, and cotton was 400, 510, and
435 mm and accounted for 31%, 40%, and 34% of annual rainfall, respectively, over the 100-year period. Early seeding can
allow crops to receive more early season rainfall and reduce the number of days of water stress during the growing season.
Average crop evapotranspiration of soybean, corn, and cotton was 546, 588, and 552 mm, respectively. Rainfall was found to
be insufficient to meet the crop water requirements beginning in the 5th, 7th, and 6th weeks after planting. Weekly effective
rain water deficits greater than 20 mm were found in weeks 7 through 16 (22 to 29 mm) for soybean, in weeks 11 through 20
(20 to 26 mm) for corn, and in weeks 11 through 16 (20 to 30 mm) for cotton. The RZWQM2 model estimated a range of annual
effective rain water deficits in soybean of 0 to 622 mm, with an average deficit of 340 mm. Deficits in corn ranged from 32 to
685 mm, with an average deficit of 355 mm, while deficits in cotton ranged from 169 to 650 mm, with an average deficit of 395
mm. Results obtained from the 100-year historical weather data can be applied to improve irrigation scheduling, water resources
planning, and design of irrigation and cropping systems in the Mississippi Delta.