ROOT-SYSTEM PARAMETERS DETERMINING WATER-UPTAKE OF FIELD CROPS

The distribution of a crop rooting system can be defined by root length density (RD), root length (RL) per soil layer of depth DELTA-z, sum of root length (SRL) in the soil profile (total root length) or rooting depth (z(r)). The combined influence of these root system parameters on water uptake is not well understood. In the present study, field data are evaluated and an attempt is made to relate a daily "maximum water uptake rate" (WU(max)) per unit soil volume as measured in different soil layers of the profile to relevant parameters of the root system. We hypothesize that local uptake rate is at its maximum when neither soil nor root characteristics limit water flow to, and uptake by, roots. Leaf area index and the potential evapotranspiration rate (ET(p)) are also important in determining WU(max), since these quantities influence transpiration and hence total crop water uptake rate. Field studies in Germany and in Western Australia showed that WU(max) depends on RD. In general, there was a strong correlation between the maximum water uptake rate of a soil layer (LWU(max)) normalized by ET(p) and RL normalized by SRL. The quantity LWU(max).ET(p)-1 was linearly related to (RL/SRL)1/2. The data show that the single root model will not predict the influence of RD on WU(max) correctly under field conditions when water-extracting neighboring roots may cause non-steady-state conditions within the time span of sequential observations. Since the rooting depth z(r) was linearly related to (SRL)1/2, the relation: LWU(max).ET(p)-1 = f (RL1/2/z(r)) holds. Furthermore it was found that the maximum "specific" uptake rate per cm root length UR(max) was inversely related to RD1/2 and to SRL1/2 or z(r) of the profile. Observed high specific uptake rates of shallow rooted crops might be explained not only by their lower RD-values but also by the additional effect of a low z(r). The relations found in this paper are helpful for realistically describing the "sink term" of dynamic water uptake models.