On inclusion of water resource management in Earth system models – Part 2: Representation of water supply and allocation and opportunities for improved modeling

Abstract

Abstract. Human water use has significantly increased during the recent past. Water withdrawals from surface and groundwater sources have altered terrestrial discharge and storage, with large variability in time and space. These withdrawals are driven by sectoral demands for water, but are commonly subject to supply constraints, which determine water allocation. Water supply and allocation, therefore, should be considered together with water demand and appropriately included in Earth system models to address various large-scale effects with or without considering possible climate interactions. In a companion paper, we review the modeling of demand in large-scale models. Here, we review the algorithms developed to represent the elements of water supply and allocation in land surface and global hydrologic models. We note that some potentially important online implications, such as the effects of large reservoirs on land-atmospheric feedbacks, have not yet been fully investigated. Regarding offline implications, we find that there are important elements, such as groundwater availability and withdrawals, and the representation of large reservoirs, which should be improved. We identify major sources of uncertainty in current simulations due to limitations in data support, water allocation algorithms, host large-scale models as well as propagation of various biases across the integrated modeling system. Considering these findings with those highlighted in our companion paper, we note that advancements in computation and coupling techniques as well as improvements in natural and anthropogenic process representation and parameterization in host large-scale models, in conjunction with remote sensing and data assimilation can facilitate inclusion of water resource management at larger scales. Nonetheless, various modeling options should be carefully considered, diagnosed and intercompared. We propose a modular framework to develop integrated models based on multiple hypotheses for data support, water resource management algorithms and host models in a unified uncertainty assessment framework. A key to this development is the availability of regional-scale data for model development, diagnosis and validation. We argue that the time is right for a global initiative, based on regional case studies, to move this agenda forward.

On inclusion of water resource management in Earth system models – Part 1: Problem definition and representation of water demand

“…The available global simulations under current conditions are compared and summarized in and Chaturvedi et al (2013a, b) for irrigative demands and in Alcamo et al (2003) and Hejazi et al (2013b) for total water consumption. Although incorporating water demand calculations can generally result in more realistic river discharge simulations (see Ngo-Duc et al, 2005a, current simulations exhibit large differences in estimates of water demand and use at countrywide, continental and global scales. This can be referred to the differences in data support, demand calculation schemes and host modelssee the discussion of Sect.…”

“…In this paper and a companion paper (hereafter Nazemi and Wheater, 2015), we focus on human-water activities manifested through water resource management and note that this is subject to operational and policy constraints. We only consider water quantity aspects of water resource management, which we define as a suite of anthropogenic activities related to storage, abstraction and redistribution of available water sources for various human demands.…”

“…In comparison to offline applications, online simulations are still at a relatively early stage of development; they typically only include irrigation, mainly implemented at regional scale and under current conditions, and present rather contradictory results. Offline applications in contrast include both irrigative and nonirrigative demands, performed under current and future con- (New et al, 1999(New et al, , (2011(New et al, , 2014 and livestock as a function of temperature 2000)…”

“…To assess the impacts of water resource management on land-surface processes and associated feedbacks with climate, the elements of water demand and water allocation should be described using computational algorithms and included in large-scale models. For the purpose of our survey, and reflecting the state of algorithm development and data availability, we focus in this paper only on the representation of water demand, and in the Nazemi and Wheater (2015) on water supply and allocation. Here, we classify human water demands under two general categories, namely irrigative and non-irrigative, and further divide nonirrigative demands into municipal, industrial, environmental, energy-related, and livestock water needs.…”

“…At this stage of model development, however, it is still unclear how operational policies should best be reflected at larger scales. At the local scale, detailed information on physical and operational systems as well as climate and water supply conditions are available (or can be generated as scenarios; see, e.g., Nazemi et al, 2002Nazemi et al, , 2013Nazemi and Wheater, 2014a, b) and the competition between demands is often reflected as an optimization problem. As the simulation scale moves from local and small basin scales to regional and global scales, the data availability degrades considerably and the high level of calculations within optimization algorithms cannot be maintained, due to computational barrier.…”