Quantifying components of the energy balance of a maize and bean intercrop

English: Quantification of the energy balance components is useful to enable analysis of irrigation scheduling and water use efficiencies in addition to calibration and validation of crop models. An experiment was conducted at Bainsvlei, Bloemfontein during the rainy season (January to April) of 2003 to quantify the energy balance components and determine the phenology of maize and bean crops within the intercropping system. The Bowen ratio energy balance method was used to quantify the components of the energy balance. Net radiant flux (Rn, soil heat flux (G), wet and dry bulb temperature and other meteorological variables were measured. The energy balance components were estimated from day of year (DOY) 47 to 79 and classified based on the height of instrument in four datasets as follows:- from DOY 47 to 58, 59 to 69, 70 to 79 and 90 to 100. During the last period (90 to 100 DOY) measurements were made over bare soil, as the crop was harvested following a hailstorm. Data collected on Cloudy days and where β approached -1 were excluded from the analysis. It was found that the latent heat flux was low throughout the crop growing season. This was mainly due to soil water stress, rather than energy availability. More than 57% of the energy was utilized in generating sensible heat flux rather than for evapotranspiration. Average net radiation inputs were 1.60±0.31 MJ m-2, 1.69±0.15 MJ m-2 , 1.19±0.57 MJ m-2 and 1.20±0.20 MJ m-2respectively, for the datasets as classified above. The average latent heat fluxes as a fraction of the net radiation during these times were 0.30Rn, 0.37Rn, 0.41Rn and 0.65Rn. The average sensible heat fluxes as a fraction of the net radiation were 0.56Rn, 0.50Rn, O.48Rn and O.22Rn for datasets as mentioned above respectively. The soil heat flux was on average 13.2% of net radiation throughout the time of measurement. Comparison of ET calculated from Bowen ratio and FAO Penman-Monteith equation showed significant difference for the hourly values. However, there was no significant difference at the daily time scale. This suggests that the methods might be complimentary for estimating ET for a long period of time, using the range of a day or more. The phenology of maize and bean was monitored during the vegetative period. It was found that the maize accumulated 90 °Cd from planting to emergency, 408 °Cd from emergency to tassel initiation and 258 °Cd from tassel initiation to vegetative stage and 243 °Cd from Rl (50% of plants have at least one flower at any node) to R4 (50% of plants have pods with seeds at beginning of pod filling stage). The maize exhibits a delay in its development even if the required growing degree-days were accumulated due to the severe water stress. However the development of the bean was not affected by the competition involved in the intercropping system both for water and light.