The risk analysis of an irrigation water allocation strategy based on physical mechanisms is critically important in practice. Conventional risk analysis only considers the role of the channel system and ignores the factors related to on-farm ponds. This paper proposes a channel-pond joint water supply mode (CPJM) based on copula approaches. Two copulas, the Plackett copula and No.16 copula, are chosen and two types of analyses are carried out with the proposed mode: (1) a risk assessment of CPJM with joint probability and conditional probability; and (2) determination of the water supply strategy given the pond water supply frequency. With a case study of the second channel in the Zhanghe Irrigation District (ZID), Southern China, nine combinations of channel water supply frequency (CWSF) and pond water supply frequency (PWSF) are studied. The results reveal that the failure probabilities of the joint distribution and the conditional distribution of the CPJM are 0.02%–16.54% and 0.45%–33.08%, respectively, with corresponding return period of 42–5000 and 10–222 years. Nevertheless, a previous study has shown that the real probability is 33.3%, which means that the return period is equals to three years. Therefore, the objective failure evaluation of the irrigation water-use strategy is useful for water saving in this channel system. Moreover, the irrigation water allocation strategy can be determined and the failure charts relating the CWSF and PWSF can be obtained for a predetermined PWSF. Thus, the channel-pond joint water supply mode provides a more reasonable estimate of the irrigation water allocation strategy reliability.

The complexity of agricultural water management practices has challenged researchers attempting to determine how hydrological pathways affect water and nitrogen (N) losses. Field experiments in a typical rice agricultural catchment in the Zhanghe Irrigation District in China were conducted in 2014 and 2015 to evaluate and quantify the connections between hydrological pathways and water and N losses. Four water management periods—namely, the irrigation period (IP), the rainfall period (RP), the normal-drainage period (DP), and the nonirrigation-drainage period (NP)—were defined based on climate conditions and artificial irrigation and drainage periods. The experimental results indicated that most water and N losses occurred in DP and RP and that lateral seepage and tailwater were the main sources of the observed drainage. Low average concentrations of ammonia N (${\mathrm{NH}}_4^{+}─\mathrm{N}$ , $0.373\mathrm{mg}{\mathrm{L}}^{-1}$ ) and nitrate N (${\mathrm{NO}}_3^{-}─\mathrm{N}$ , $0.285\mathrm{mg}{\mathrm{L}}^{-1}$ ) and low proportions of dissolved N (42.2–54.0%) were detected in association with green manure application, whereas the average concentration of total N (TN, $1.376\mathrm{mg}{\mathrm{L}}^{-1}$ ) was relatively high, which resulted in slight N pollution in the studied agricultural area. The concentrations and proportions of dissolved N were greastest in DP and RP. The average daily mass fluxes of ${\mathrm{NH}}_4^{+}─\mathrm{N}$ , ${\mathrm{NO}}_3^{-}─\mathrm{N}$ , and TN were 0.027, 0.020, and $0.095\mathrm{kg}{\mathrm{ha}}^{-1}{\mathrm{day}}^{-1}$ , respectively, and the magnitudes in DP and RP were significantly greater than those in IP and NP. The average concentrations of ${\mathrm{NH}}_4^{+}─\mathrm{N}$ , ${\mathrm{NO}}_3^{-}─\mathrm{N}$ , and TN were not correlated to drainage discharge according to a Pearson correlation analysis, whereas a power function can quantify the relationship between N mass fluxes and discharge during various water management periods. The average thresholds of drainage discharge, at which N export from paddy fields equalled to zero, were $45.2{\mathrm{m}}^3{\mathrm{day}}^{-1}{\mathrm{ha}}^{-1}$ for ${\mathrm{NH}}_4^{+}─\mathrm{N}$ , $51.9{\mathrm{m}}^3{\mathrm{day}}^{-1}{\mathrm{ha}}^{-1}$ for ${\mathrm{NO}}_3^{-}─\mathrm{N}$ , and $55.5{\mathrm{m}}^3{\mathrm{day}}^{-1}{\mathrm{ha}}^{-1}$ for TN. These findings indicate that controlling drainage discharge is important for the reduction of water loss and N loss from paddy fields.