• Determining Soil Moisture for Irrigation Management

      Martin, E. C.; Husman, S.; Wegener, R.; Brown, P.; Johnson, K.; Schnakenberg, L.; Silvertooth, Jeff (College of Agriculture, University of Arizona (Tucson, AZ), 1995-03)
      One key component in good irrigation management is the measurement of soil moisture to help determine when to irrigate. In this study, resistance blocks and tensiometers were compared to neutron probe readings to assess how well these devices followed soil moisture and whether the resistance blocks and /or tensiometers could be used to schedule irrigation in cotton production. The resistance blocks were placed at 6, 18, and 30 inches. Tensiometers were placed at 18 and 30 inches. The readings from the resistance blocks and tensiometers were compared to neutron probe readings taken at 6, 18, and 30 inches. The resistance blocks compared well with the neutron probe readings at the 6 inch and 30 inch depth. At the 18 inch depth, there was much scatter in the data. The tensiometers also showed good comparisons at 30 inches and poor comparisons at 18 inches.
    • Upland Cotton Water Stress Sensitivity by Maturity Class and Suggesting Management Strategy

      Husman, S.; Wegener, R.; Brown, P.; Martin, E.; Johnson, K.; Schnakenberg, L.; Silvertooth, Jeff (College of Agriculture, University of Arizona (Tucson, AZ), 1995-03)
      Lint yield response to differing irrigation treatments based on maximum allowable soil moisture depletions was tested in an indeterminate (D +PL 5816) and a determinate variety (D+PL 5415) selection. The Arizona Meteorological Weather Network (AzMet) was used to summate evapotranspiration demands with irrigations triggered at 35 (wet), 50 (med), and 65 % (dry) maximum allowable soil moisture depletion levels. Soil water holding capacity was gravimetrically measured to a depth of four feet on one foot increments. The study consisted of three treatments replicated four times utilizing a complete block split plot design. When the allowable depletion level was attained, the water volume necessary to refill the effective root zone was delivered. This was accomplished by manipulating irrigation set times and flow rates. Irrigation volumes were 66.7, 57.2, and 46.9 acre - inches for the wet, medium, and dry treatments respectively. Lint yields were significantly reduced when the maximum allowable soil moisture depletion exceeded 50% in the determinate variety selection while there were no significant lint yield differences in any of the irrigation treatments with the more indeterminate variety. Water stress sensitivity is increased with the determinate variety while the indeterminate variety is more forgiving. With variety selection shifts in the Central Arizona desert towards a reduced season approach and utilization of more determinate varieties, water management strategy should be modified to minimize or eliminate any water stress during the flowering period. The AzMet weather network information offers cost effective (free) and reliable water use information. The system can be used to assist with irrigation scheduling if a producer is willing to attempt to characterize differing soil water holding capacities on the farm and manage accordingly.