New indexes to evaluate the effect of segmental variations of distributed ditches on their pollutant retention in agricultural landscapes

Distributed drainage ditches and ponds in agricultural landscape can reduce chemicals export from farmlands; their distributed features, however, result in variations in pollutant removal capacities among different segments. In this paper, we proposed three descriptive indexes to characterize segmental variations of distributed ditches and ponds systems: a size index (α - segmental size vs. system size) to represent the static feature of the system, a flow index (β - segmental flow rate vs. system flow rate) and an initial concentration index (γ - segmental inflow concentration vs. initial field drainage concentration) to represent the dynamic feature of the system. Subsequently, a segmental efficiency indicator (η) was derived from these indexes to represent the pollutant removal differences calculated with and without considering segmental variations of distributed ditch systems. The proposed indexes were tested with two hypothetic ditch layouts first, and then applied to a case study site where 18 segments of ditches and ponds are distributed around paddy fields in southeastern China. The two hypothetical layouts include a ‘slender’ distribution and a ‘square’ distribution of ditches; both have the same number of identical segments and serve the same area. The ‘slender’ layout represents more spread distributions while the ‘square’ layout represents more concentrated systems. Calculated indexes showed that the ‘slender’ layout produced greater segmental variations than the ‘square’ layout. For the case study, the calculated size index and the flow index varied greatly due to segmental size differences and the complex flow connections. This in turn led to greater variation in the pollutant removal efficiencies (η), ranging from 0.021 to 0.63 for low removal rate, and 0.021–0.42 for high removal rate. The results indicate that the proposed indexes can be used to examine the variable nature of the distributed ditch systems and help provide technical guidance for proper management of the distributed buffer systems in agricultural landscapes.