Numerical HYDRUS 2D simulation of root zone water uptake dynamics by drip irrigated asparagus

Asparagus (Asparagus officinalis L.) is a perennial vegetable grown in the north eastern part of Belgium. Asparagus is grown on sandy soils with low water holding capacity. Asparagus has been reported to be sensitive to drought stress, with reduced stomatal conductivity and assimilation rate at high soil and plant water potential. On the other hand, the root system has been known to be extensive, which should permit water extraction from a large soil volume. In 2015, a field experiment was conducted to gain insight into the dynamics of root zone water uptake by asparagus under three different drip irrigation treatments; a full irrigation treatment at which irrigation was initiated at -60 kPa, a treatment, which received 50% of irrigation water was compared to the full irrigation treatment and a rain-fed treatment. In every irrigation treatment, crop development and gravimetric soil moisture observations were collected in four replications. For each treatment, one plot was equipped with 6 Watermark granular matrix sensors to observe the variation in soil water potential (Ψsoil). In the autumn, the Watermark sensors were removed and root samples collected at the same place to determine the fine root distribution. The acquired root distribution, in combination with in situ observed water retention characteristics and saturated hydraulic conductivity was fed into the HYDRUS 2D model to numerically describe the water extraction pattern of asparagus. With this calculation, the relation between soil water potential and plant water status was identified in each drip irrigation treatment. Although calculated Ψsoil agreed only moderately with observed Ψsoil, the calculation suggested that transpiration declined in rain-fed asparagus, despite its extensive root system.