On the importance of including vegetation dynamics in Budyko's hydrological model

128

112

Abstract. Groundwater-dependent ecosystems (GDEs) are at risk globally due to unsustainable levels of groundwater extraction, especially in arid and semi-arid regions. In this review, we examine recent developments in the ecohydrology of GDEs with a focus on three knowledge gaps: (1) how do we locate GDEs, (2) how much water is transpired from shallow aquifers by GDEs and (3) what are the responses of GDEs to excessive groundwater extraction? The answers to these questions will determine water allocations that are required to sustain functioning of GDEs and to guide regulations on groundwater extraction to avoid negative impacts on GDEs.We discuss three methods for identifying GDEs: (1) techniques relying on remotely sensed information; (2) fluctuations in depth-to-groundwater that are associated with diurnal variations in transpiration; and (3) stable isotope analysis of water sources in the transpiration stream.We then discuss several methods for estimating rates of GW use, including direct measurement using sapflux or eddy covariance technologies, estimation of a climate wetness index within a Budyko framework, spatial distribution of evapotranspiration (ET) using remote sensing, groundwater modelling and stable isotopes. Remote sensing methods often rely on direct measurements to calibrate the relationship between vegetation indices and ET. ET from GDEs is also determined using hydrologic models of varying complexity, from the White method to fully coupled, variable saturation models. Combinations of methods are typically employed to obtain clearer insight into the components of groundwater discharge in GDEs, such as the proportional importance of transpiration versus evaporation (e.g. using stable isotopes) or from groundwater versus rainwater sources.Groundwater extraction can have severe consequences for the structure and function of GDEs. In the most extreme cases, phreatophytes experience crown dieback and death following groundwater drawdown. We provide a brief review of two case studies of the impacts of GW extraction and then provide an ecosystem-scale, multiple trait, integrated metric of the impact of differences in groundwater depth on the structure and function of eucalypt forests growing along a natural gradient in depth-to-groundwater. We conclude with a discussion of a depth-to-groundwater threshold in this mesic GDE. Beyond this threshold, significant changes occur in ecosystem structure and function.

Section: Eamus Et Al: Groundwater-dependent Ecosystems: Recent Inmentioning

confidence: 99%

Groundwater-dependent ecosystems: recent insights from satellite and field-based studies

Eamus

Zolfaghar

Villalobos‐Vega

et al. 2015

128

112

“…This curve has been examined by a number of modeling and observational studies (Milly, 1994;Zhang et al, 2001;Atkinson, 2002;Farmer et al, 2003;Porporato et al, 2004;Donohue et al, 2007;Yokoo et al, 2008;Gentine et al, 2012;Troch et al, 2013) that have elucidated the first-order controls on this partitioning. These suggest that for a given ratio of Ep/P the variation between watersheds (that is, the scatter around the curve) is largely controlled by a number of factors that can be summarized as follows:…”

Section: Water Balancementioning

confidence: 99%

What makes Darwinian hydrology "Darwinian"? Asking a different kind of question about landscapes

Harman

Troch

2014

Abstract. There have been repeated calls for a Darwinian approach to hydrologic science, or for a synthesis of Darwinian and Newtonian approaches, to deepen understanding of the hydrologic system in the larger landscape context, and so develop a better basis for predictions now and in an uncertain future. But what exactly makes a Darwinian approach to hydrology "Darwinian"? While there have now been a number of discussions of Darwinian approaches, many referencing Harte (2002), the term is potentially a source of confusion because its connections to Darwin remain allusive rather than explicit.Here we suggest that the Darwinian approach to hydrology follows the example of Charles Darwin by focusing attention on the patterns of variation in populations and seeking hypotheses that explain these patterns in terms of the mechanisms and conditions that determine their historical development. These hypotheses do not simply catalog patterns or predict them statistically -they connect the present structure with processes operating in the past. Nor are they explanations presented without independent evidence or critical analysis -Darwin's hypotheses about the mechanisms underlying present-day variation could be independently tested and validated. With a Darwinian framework in mind, it is easy to see that a great deal of hydrologic research has already been done that contributes to a Darwinian hydrologywhether deliberately or not.We discuss some practical and philosophical issues with this approach to hydrologic science: how are explanatory hypotheses generated? What constitutes a good hypothesis? How are hypotheses tested? "Historical" sciences -including paleohydrology -have long grappled with these questions, as must a Darwinian hydrologic science. We can draw on Darwin's own example for some answers, though there are ongoing debates about the philosophical nature of his methods and reasoning. Darwin used a range of methods of historical reasoning to develop explanatory hypotheses: extrapolating mechanisms, space for time substitution, and looking for signatures of history. Some of these are already in use, while others are not and could be used to develop new insights. He sought explanatory hypotheses that intelligibly unified disparate facts, were testable against evidence, and had fertile implications for further research. He provided evidence to support his hypotheses by deducing corollary conditions ("if explanation A is true, then B will also be true") and comparing these to observations. While a synthesis of the Darwinian and Newtonian approaches remains a goal, the Darwinian approach to hydrologic science has significant value of its own. The Darwinian hydrology that has been conducted already has not been coordinated or linked into a general body of theory and knowledge, but the time is coming when this will be possible.

“…When estimating annual water yield or annual ET by this equation, we assume that the change in soil water storage ( S) is negligible for normal years, but may cause large errors in extreme climatic conditions (Donohue et al, 2007). Fortunately, the streamflow of Loess Plateau region is dominated by surface runoff, and annual changes of soil water and groundwater are not likely to be large in order to cause a concern for annual estimate of ET by the water balance method.…”

Section: Monthly Et and Annual Water Yield (Q) Estimatesmentioning

confidence: 99%

Regional effects of vegetation restoration on water yield across the Loess Plateau, China

Feng

Sun

et al. 2012

188

Abstract. The general relationships between vegetation and water yield under different climatic regimes are well established at a small watershed scale in the past century. However, applications of these basic theories to evaluate the regional effects of land cover change on water resources remain challenging due to the complex interactions of vegetation and climatic variability and hydrologic processes at the large scale. The objective of this study was to explore ways to examine the spatial and temporal effects of a large ecological restoration project on water yield across the Loess Plateau region in northern China. We estimated annual water yield as the difference between precipitation input and modelled actual evapotranspiration (ET) output. We constructed a monthly ET model using published ET data derived from eddy flux measurements and watershed streamflow data. We validated the ET models at a watershed and regional levels. The model was then applied to examine regional water yield under land cover change and climatic variability during the implementation of the Grain-for-Green (GFG) project during 1999-2007. We found that water yield in 38 % of the Loess Plateau area might have decreased (1-48 mm per year) as a result of land cover change alone. However, combined with climatic variability, 37 % of the study area might have seen a decrease in water yield with a range of 1-54 mm per year, and 35 % of the study area might have seen an increase with a range of 1-10 mm per year. Across the study region, climate variability masked or strengthened the water yield response to vegetation restoration. The absolute annual water yield change due to vegetation restoration varied with precipitation regimes with the highest in wet years, but the relative water yield changes were most pronounced in dry years. We concluded that the effects of land cover change associated with ecological restoration varied greatly over time and space and were strongly influenced by climatic variability in the arid region. The current regional vegetation restoration projects have variable effects on local water resources across the region. Land management planning must consider the influences of spatial climate variability and long-term climate change on water yield to be more effective for achieving environmental sustainability.