Parameterizing field capacity as the upper limit of available water in bucket-type hydrological models

Abstract

• FC was calibrated in a bucket model (BM) to best mimic a Richards equation-based model. • FC calibrated for BM depended on soil, crop, and was strongly influenced by climate type. • FC calibrated for BM was more than one pF-unit closer to saturation than what is usually considered. • Pressure heads commonly used for FC underestimated crop available water in BM. Field capacity (FC) was originally defined as a soil profile property without relation to crop water uptake. Nevertheless, it has been frequently used as the upper limit for available water in bucket-type models. We aimed to find the value of FC in the bucket-type model WOFOST that best simulates the water balance predicted by the Richards equation-based model SWAP, here considered as a benchmark. With an equal description of plant growth, we calibrated FC in WOFOST by minimizing the root mean square deviation between selected outputs (targets) from WOFOST and SWAP. The calibrated FC value depended on the target, soil type, climate, and crop drought sensitivity. Using common soil-based values for FC, available water was underestimated up to 60%. Crop production parameters were insensitive to FC, and although best values for FC were close to saturation, crop production results were similar to employing a common pF value between 1.0 and 2.0.