By neglecting unsaturated flow, Wu et al. (2018, https://doi.org/10.1029/2018WR023070) claimed that the zero volumetric discharge at the upstream boundary results in two possible boundary conditions of either zero groundwater table or zero seepage velocity based on the original Dupuit‐Boussinesq theory. They further concluded that a steeper and/or shallower aquifer has a zero‐groundwater table if the downstream groundwater table is shallower. Here we point out that the conceptual model adopted by Wu et al. (2018) not reasonable and the results are quite different from those with unsaturated flow considered in the real hillslope aquifer. Specifically, the water table at the upstream boundary could be positive or negative due to the lateral unsaturated flow effect, and hence, the seepage velocity must be set to zero to satisfy the zero discharge boundary condition at the upstream boundary. In addition, the zero groundwater table cannot ensure a zero discharge at the upstream boundary. Furthermore, we argue that the discharge with respect to the groundwater table is linearly distributed along the slope distance only when both saturated and unsaturated flows are considered. The saturated flow discharge itself is not distributed linearly. Our comment highlights the importance of unsaturated flow in studying steady groundwater flow in an unconfined sloping aquifer with uniform recharge using the Dupuit‐Boussinesq theory.
The spatial variability of carbon dioxide (CO 2 ) and methane (CH 4 ) fluxes across water-air interface in Xuanwu Lake was investigated in two seasons. Due to anthropogenic disturbances, the environmental factors and the fluxes of CO 2 and CH 4 in lake showed obvious spatial and seasonal variability; their average fluxes in summer are significantly higher than those in autumn. The fluxes in heavy pollution sites with high concentrations of nitrogen and phosphorus nutrient in summer were 3.9 times (142.14 : 36.07 mg⋅m −2 ⋅h −1 ) for CO 2 and 22.3 times for CH 4 (6.46 : 0.29) higher than those in little pollution sites. In autumn, they were 12.3 times and 7.1 times higher, respectively. Anthropogenic disturbance and heavy pollution increased their fluxes, but aquatic plants reduced the emission of CO 2 . Except the sampling site with flourishing lotus, most of sampling sites without aquatic plant are the emission source of CO 2 and CH 4 . The correlation analysis, multiple stepwise regression, and redundancy analysis showed the key environmental factors for CO 2 including temperature (T), pH, chemical oxygen demand (COD Mn ) in water, organic matter (OM), total nitrogen, and ammonia nitrogen in water and sediment. As for CH 4 , the key environmental factors include turbidity, oxidation-reduction potential, dissolved oxygen, COD Mn , and T in water and OM and N-NH 4 + in sediment.
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