|The authors have adequately addressed many concerns posed in the first review. The restructuring of the paper (describing theory first, followed by a case study MDB example) is effective. The paper flows and is easier to read. |
However, the main issue identified in the first review has not been sufficiently addressed.
Original comment: The authors claim their proposed water balance is an improved management tool to balance water needs of humans vs. ecosystem needs. However, their structuring of the water balance retains a fundamental flaw in human thinking about ecosystem needs for water. In the authors’ rendering of water accounting, all water needs are accounted for, leaving aquatic ecosystem requirements as a “whatever is left” term. Page 917, line 11 (now page 6, lines 16-18 in the revised manuscript) “ . . .the remaining surface runoff is retained for ecosystem purposes or flows into the sea.” This conceptualization supports outdated and disregarded ideas that water flowing in the river to the sea is ‘wasted’ or that human uses of water may continue unabated until all flows are appropriated. The proposed places unbalanced emphasis on terrestrial water needs while ignoring aquatic needs. The authors thus demonstrate little understanding of ecosystem services provided by aquatic ecosystems and the role of flow regime in aquatic ecosystem function, both of which should ideally be addressed in their framework. To remedy, the authors could define Rout in Eq. 1 as river runoff. The Rout term can then be unpacked as follows: Rout = Re + Roth Where Re is a term for ecological river flows and Roth is what remains after ecosystem needs and human needs have been accounted for. Rather than explaining Re as runoff to the sea, suggesting it holds little or no benefit to the river basin, the authors may state that this quantity must be maintained at specified values through the water year to support ecosystem services in quantities determined through environmental flows assessment in accordance with the natural flow regime of the river basin.
Authors’ response: Thanks for this very valuable critique. We fully agree that the role of flow regime in aquatic ecosystem function should be addressed in the framework. Due to the data limitations, in this proposed socio-hydrological water balance framework, ecological system evapotranspiration includes evapotranspiration from precipitation, surface runoff, and groundwater in native vegetation areas. Water consumed in aquatic ecosystems to support ecosystem services was considered a part of the evapotranspiration from surface runoff. However we have been aware that, in addition to GPP of native vegetation systems, the size and quality of inland wetlands and riverine ecosystems and aquatic ecosystems should be considered to assess the impacts of water allocation on catchment ecosystems. These limitations of this study have been explained in the Discussion section.
The authors have not attempted to address this concern, to the extent that even the sentence quoted directly in the original comment has not been revised.
I maintain that it is not possible to disregard river flows in discussions of water allocation between human and ecosystem needs. If the authors wish to focus only on terrestrial water needs, or if, as the authors state, their data limitations are so severe as to limit the scope of their analysis, the stated objective of their work must reflect this. However, the authors state that their objective is “…to develop a simple socio-hydrological water balance framework for allocating water between human society and the environment to support sustainable water management of catchments.” A catchment water balance intended to call attention to human-ecological balances simply cannot neglect or minimize the importance of ecological services/functions provided by surface water flows.
The authors’ rationale for doing so seems to stem from an assumption that the value of an ecosystem function is somehow equateable with the volume of catchment water necessary for providing that function. There is no evidence to support this assumption, and in fact, potentially much evidence to the contrary. In discussion of basin restoration efforts to buy back irrigation water for ecological river flows, the authors state that these buyback targets represent less than 1% of the water balance. However, the impact of redirecting this 1% of catchment water balance to maintain river health is likely to have great ecological significance.
I suggest that the authors either adequately address the needs of aquatic ecosystems, or revise the objectives of their manuscript to focus specifically on terrestrial ecosystems.