2.1. Description of Field Experiments
To develop key plant parameters for vegetables for the ALMANAC model, multiple field experimental plots were established at the USDA-ARS Grassland, Soil, and Water Research farm in Temple, TX, USA. Field experiments of beans, mint, pepper, squash, and cabbages were conducted in different years. The soil type at the site is a Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert). Total precipitation and maximum and minimum temperature of the growing season of each vegetable are listed in
Table 1. Supplemental irrigation was applied to ensure that the vegetable plants did not suffer any water stress. All plants were fertilized prior to planting by broadcasting 15–15–15 (nitrogen-phosphorus-potassium) fertilizer at 46 kg ha
−1. Total aboveground biomass of all plants was harvested manually every two weeks throughout the growing season. At each harvest, the heights for all plants were measured from the ground to top of the highest leaf. Photosynthetically Active Radiation (PAR) was measured below the leaf canopy using an ACCUPAR LP-80 Ceptometer (Decagon Devices, Pullman, WA, USA). The PAR value was used to calculate the fraction of intercepted PAR (FIPAR). The light readings were made between 10:00 and 14:00. Fresh total aboveground biomass weight, seeds, and dead leaves were measured. The leaf area (cm
2) for each plant sample was measured with a LI-3100 Area Meter (LI-COR Biosciences, Lincoln, NE, USA). Based on the leaf area, fresh weight, and sampling area, leaf area index (LAI) of each plant sample was calculated. The light extinction coefficient (k) was calculated as the natural log of difference between 1 and FIPAR and then divided by LAI. Following these measurements, the fresh samples were dried at 66 °C until dry weight stabilized, after which, the dry weights of each sample were measured. Detailed information on plot design, planting date, and harvesting date for each experiment is given below. The year and month of growing season of each vegetable are summarized in
Table 1.
Study 1: In Study 1, one cultivar of bush beans and one cultivar of green beans were planted. In 2014, bush beans were planted at 90 kg per ha, two rows were 0.95 m apart with six to eight seeds per 0.3 m of row. Harvests were performed on September 25th, October 8th, October 24th, and November 4th. In 2016, green beans were planted at 90 kg per ha, in two rows 0.95 m apart at six to eight seeds per 0.3 m of row on May 1st. Harvests were done on May 25th, June 8th, June 21st, and July 7th.
Study 2: In 2015, cultivars of peppermint and spearmint were transplanted into plots on April 30th and May 15th, respectively. Four-week-old mint seedlings were purchased from the local feed store. Each plot consisted of three 3 m long rows, spaced 0.75 m apart, with 0.35 m between plants within rows, resulting in a plant density of 5.5. plants m−2. Harvests for each mint cultivar were done on May 6th, May25th, June 8th, June 21st, July 8th, and July 25th.
Study 3: In Study 3, cabbage (Brassica oleracea var. capitata) was planted as four-week-old cabbage seedlings purchased from a local feed store and transplanted into plots on October 1st, 2014. Each plot consisted of three 3 m long rows, spaced 0.75 m apart, with 0.35 m between plants within rows, resulting in a plant density of 5.5. plants m−2. Harvests were done on October 17th, October 31st, November 17th, December 5th, December 16th, and January 15th, 2015.
Study 4: In Study 4, two cultivars of squash, straight neck, and zucchini, were planted as four-week-old seedlings that were purchased from a local feed store and were transplanted into plots on June 15th, 2016. Plots consisted of three 3 m long rows of each of squash, spaced 0.75 m apart, with 0.35 m between plants within rows. Harvests were done on June 29th, July 5th, July 14th, July 21st, and July 25th for zucchini, while a total of eight harvests were done on June 29th, July 5th, July 14th, July 21st, July 25th, August 3rd, August 11th, and August 30th for straight neck.
Study 5: In Study 5, one cultivar of bell pepper was planted as four-week-old pepper seedlings that were purchased from a local feed store and transplanted into plots on June 1st, 2017. Each plot consisted of three 3 m long rows of each squash, spaced 0.75 m apart, with 0.35 m between plants within rows. Harvests were performed on June 13th, June 23rd, July 6th, July 18th, and July 26th.
2.5. Description of Study Irrigation Plots in Winter Garden Region in TX, USA
Eight irrigated locations were randomly selected from the Dimmit, La Salle, and Frio counties in the Winter Garden Region. All irrigated plots are classified as either Irrigated Capability Class 1 or 2, where soils have either no or moderate limitations that reduce the choice of plants or that require moderate conservation practices (NRCS-USDA, 2018). Soil types and their properties in the study locations are listed in
Table A1. We developed and modeled 960 scenarios that represent projected vegetable production management conditions under different combinations of five irrigation rates, eight vegetable types, three years (2013, 2017, and 2019) and eight locations using the calibrated ALMANAC model. The five irrigation rates were 0, 50, 80, 100, and 150% of calculated crop evapotranspiration (ET) of each species in each study location. We used the same crop management that was used for model calibration and validation. The total rainfall amounts in mm during growing months in the Dimmit, La Salle, and Frio counites are listed in
Table A2.
Based on the simulation results of 960 scenarios, analysis of variance (ANOVA) was used to statistically analyze the effects of treatment (irrigation regime) on the simulated marketable dry yield and water use efficiency (WUE). The Least Significant Difference test (LSD) was used to compare and rank the treatments. Simulated marketable dry yield under five irrigation rates (0, 50, 80, 100, and 150%) was analyzed for the eight vegetables. To accurately estimate monetary profit according to different water use, water cost was computed based on the water service fee in the Winter Garden Region of Texas [
6]. Value creation rate (i.e., wet yield per water cost) was also analyzed. Logarithmic models were used to understand the trend of value creation rates of the eight vegetables. In addition, the profit of each vegetable was calculated by subtracting total irrigation water cost from revenue. The retail prices for bush bean, green bean, cabbage, yellow straight neck squash, zucchini, and bell pepper were obtained from USDA-Economic Research Service [
7]. Spearmint and peppermint were excluded for profit analysis since their retail prices were not available from the USDA database. The revenue cost for each vegetable was obtained by multiplying total marketable wet yield with revenue cost. This study only considered water cost in order to understand precisely the impact of water use on vegetable profit.