Penman’s yield response model and infrared thermometry for scheduling irrigation of semi-leafless peas and beans

The response of a semi-leafless Pisum sativum (Field pea, C.V., Rovar x SL.) to irrigation at different growth stages (vegetative, flowering, podset) and at different soil moisture contents was investigated in a field experiment at Lincoln College during the 1982/83 season. The crop was grown on a Templeton silt loam. When irrigation was applied it was to replace the difference between potential evapotranspiration adjusted for crop cover and rainfall for the previous week. The concept of moisture sensitive stages and Penman's yield response model to irrigation and potential soil moisture deficit was evaluated. Measurements of solar radiation interception, soil moisture content and plant variables were measured to help determine how drought and irrigation influenced crop yield. The utility of infrared thermometry to determine if crop canopy temperature (Tc) could be used to detect crop moisture stress and for scheduling irrigations of pea and bean crops was carried out during the 1982/83 and 1983/84 seasons at Lincoln College. Irrigation increased yields at all stages of growth with no clear evidence of moisture sensitive developmental periods. There was, in general, a linear response of total dry matter, grain yield, yield components, morphological characteristics, and crop absorption of photosynthetically active radiation (PAR) to water received by the crop, independent of when the water was received, and a linear decline of these characteristics with increasing maximum potential soil moisture deficit (Dm). Penman's model of yield response to Dm and response to irrigation provided a satisfactory framework for interpreting the effect of drought and determining irrigation need. The limiting deficit, Dl, from the December sowing was 60 mm and the October sowing 80 to 90 mm, a range of 30 to 53% of plant available water. Dl may change with time depending on when particular soil moisture deficits occur. Decline in yield relative to fully irrigated yield with increasing Dm was 0.2% of potential yield for each mm of Dm. If a crop has a potential grain yield of 5000 kg ha⁻¹ irrigating with 30 mm when the deficit exceeds the limiting deficit would produce 300 kg ha- l or 10 kg ha⁻¹ mm⁻¹. The higher the potential yield the greater the absolute response mm⁻¹. PAR conversion efficiency (U) was 1.2 g m⁻² MJ⁻¹ with no response to irrigation or Dm. This low value may in part be due to damage caused by a hail storm 20 January 1983. Grain yield increased linearly per unit of PAR absorbed (1.1 g MJ⁻¹) until 440 MJ m⁻², thereafter a progressive decline occurred to 0.03 g MJ⁻¹ at 550 MJ⁻¹ m⁻² absorbed. The change in transpiration to PAR absorbed (α) was 0.5 mm MJ⁻¹, greater than the α in the more humid environment of England. The transpiration efficiency coefficient was between 0.10 and 0.13 kPa. This lower value may also have been the result of hail damage. There was no difference between treatments in water evapotranspired to produce grain with a mean value of 8.4 kg ha⁻¹ mm⁻¹. An effective soil moisture deficit, Dₑ, was found to have promise as an estimate for assessing crop moisture stress from a soil moisture budget. Crop moisture stress was detected using a hand held infrared thermometer. Canopy resistance to vapour transfer calculated from Tc data increased with increasing Dₑ for Vicia faba and Phaseolus vulgaris. Canopy temperature based irrigation scheduling criteria evaluated were the stress degree day index (SDD), canopy temperature variability (CTV), crop water stress index (CWSI) and canopy temperature difference between stressed and unstressed crops (CTD). Dependence of the canopy temperature - air temperature difference on vapour pressure deficit (vpd) could be used under limited conditions when vpd > 2.0 kPa. The difference between the canopy temperature of a stressed and unstressed crop seems to hold most promise as being suitable for scheduling irrigations. As a scheduling criterion, however, it may have practical problems such as the maintenance of an unstressed control.