Accurate phosphorus (P) load estimation in subsurface drainage water is critical to assess the field‐scale efficacy of conservation practices. The HydroCycle‐PO4 instrument measures real‐time total reactive P (TRP) concentration without the need for sample filtration, thereby enabling comparative evaluation of different sampling strategies. The main objective of this study was to evaluate the effects of water sampling strategies on the uncertainty of P load estimation. Hourly TRP concentration and hourly drainage discharge measurements formed the reference P load dataset. Four hypothetical water sampling strategies were evaluated: (a) time‐proportional discrete sampling, (b) time‐proportional composite sampling, (c) flow‐proportional discrete sampling, and (d) flow‐proportional composite sampling. All sampling strategies underestimated TRP load compared with the reference dataset. Total reactive P load underestimation changed from 0.2 to 51% as time‐proportional discrete sampling intervals increased from 3 h to 14 d. Total reactive P load underestimation changed from 12 to 43% as the time‐proportional compositing scenario increased from 1 to 7 d, each with one aliquot per day. In the case of flow‐proportional discrete sampling scenario, the lowest (0.6%) and the highest (–5.1%) uncertainties were observed when 1‐ and 5‐mm flow intervals were used. The relative error based on the results provided by the flow‐proportional composite sampling ranged from 0.2% when using 1‐mm flow interval to –6.7% when using 5‐mm flow interval. In conclusion, the flow‐proportional sampling strategies provided a more accurate estimate of cumulative P load with fewer number of samples because a greater portion of samples were taken at higher flow rates compared with time‐proportional sampling strategies.
Surface irrigation models should take two‐dimensional infiltration of water in furrows into account for better representation and more accuracy. The Warrick–Lazarovitch infiltration model (W–L model) has been proposed to estimate the two‐dimensional infiltration of water in furrow irrigation. This model consists of several parameters, including γ as an empirical parameter, which is often obtained through calibration of the model. This study aimed to evaluate the W–L model performance in determining the cumulative and lateral infiltrations under conventional furrow irrigation (CFI) and alternate furrow irrigation (AFI) schemes and two different initial and boundary conditions. The results indicate that the W–L model adequately estimated the cumulative and lateral infiltration, and the two‐dimensional infiltration was estimated with less error than the lateral infiltration. In both irrigation schemes, for an increased surface boundary head from 5 to 10 cm and for a reduced initial water content from 0.21 to 0.16 cm3/cm3, the value of γ would increase. Generally, γ values in AFI were greater than those in CFI. The W–L model estimated the two‐dimensional infiltration with more accuracy than the relative lateral infiltration for both CFI and AFI. Proper representation of infiltration is important in both irrigation management and in tracing footprints of chemicals used in agriculture.
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