Evaluation of Evapotranspiration and Production of Paprika (Capsicum Annum L.) using the Soil Water Balance Approach under Variable Irrigation Water Applications

Precise estimation of evapotranspiration (ET) at the different growth stages is important for determining the field soil water balance and irrigation requirements. An experiment was conducted at the Research Field Station of the University of Zambia on a summer high value crop using a small-scale drip irrigation system. To maximize water use efficiency, the soil evaporation (E) and crop transpiration (T) components of the total ET were estimated for the cropping season. A randomized complete block design experiment with four replications and four irrigation water application rates based on crop water requirements (50 percent, 75 percent, 100 percent with and without plastic mulch), was carried out with paprika (Capsicum Annum L.) as a test crop. To determine T, soil water content was monitored with a soil moisture neutron probe to a depth of 150 cm over the season; green canopy cover was also monitored regularly using images taken with a digital camera. The results showed that T was negligible during the first 25 d after transplant (DAT) during plant establishment due to incomplete canopy cover and thereafter started to increase exponentially. The partitioning of ET into E and T using canopy cover values multiplied by ET showed that T estimates varied from 38 mm to 70 mm, depending on the treatments and resulted in an average value of 58.8 mm. Transpiration from the treatment with plastic mulch was 5 mm lower, with most of the water loss being through E during plant establishment before the plastic cover was installed. Generally, during the vegetative stage (the first 70 DAT) 75 percent of ET was in form of E while the remaining 25 percent was lost through T. The results on biomass production showed that the highest biomass production (421 kg/ha) was obtained in the 100 percent treatment while the lowest biomass (233 kg/ha) was in the 50 percent treatment; hence the higher the amount of water applied, the higher was the biomass produced. However, water use efficiency (biomass per unit ET) was highest under 50 percent ET and lowest in the 75 percent treatment (6 kg·ha^-1·mm^-1 versus 5.1 kg·ha^-1·mm^-1). (author)