Earth’s Critical Zone, the thin outer layer of our planet from the top of the tree canopy to the bottom of water aquifers that supports almost all human activity, is experiencing ever-increasing pressure from growth in human population, wealth and climatic changes. Understanding, predicting and managing intensification of water use and associated economic services, while mitigating and adapting to rapid climate change and biodiversity decline, is now one of the most pressing societal challenges of the 21st century. Thus, the knowledge of how vegetation affects water storage and flow pathways is essential for a more efficient and sustainable management of water resources. In spite of past efforts to assess the role of vegetation on the water cycle, a thorough understanding of the ecohydrological mechanisms according to which vegetation stores and transpires water, interacts with runoff generation and affects flow regimes is still missing. Particularly, recent works argued the truthfulness of the widely adopted paradigm of a single ecohydrological reservoir, and suggested that two ‘water worlds', one originating groundwater and stream runoff, and one associated with the vegetation water uptake, may exist. The lack of water exchange between the two soil pools provides a fundamental challenge to current conceptualizations and analyses of water-cycle processes.
The general goal of the project is to gain new insights on the water partitioning and mixing within the Earth Critical Zone by testing hypotheses of eco-hydrological separation of vegetation water use. For this, the project will couple advanced isotopic, geophysical and micro-meteorological monitoring with detailed eco-hydrological models, and will specifically focus on the Mediterranean area. Finally, the project will develop a framework to translate the new critical zone knowledge into evidence to support policy and management decisions concerning water and land use in forested and agricultural ecosystems.
The project includes the organisation a Critical Zone Observatories Network. This includes five field sites which will provide a consistent access to different climatic, hydrological and ecological conditions which are representative of the Mediterranean and Alpine-Mediterranean environments. Each Observatory involves co-located research to be conducted by inter-disciplinary teams. By testing hypotheses of eco-hydrological separation of vegetation water use across multiple sites, the project will advance our capability to predict the effects of vegetation and climate change on water availability in space and time.
1 - State of the art
Earth’s Critical Zone (CZ), the thin outer layer of our planet from the top of the tree canopy to the bottom of water aquifers that supports almost all human activity, is experiencing ever-increasing pressure from growth in human population, wealth and climatic changes. Within the next decades, global demand for food and fuel is expected to double along with a more than 50% increase in demand for clean water. Understanding, predicting and managing intensification of water use and associated economic services, while mitigating and adapting to rapid climate change and biodiversity decline, is now one of the most pressing societal challenges of the 21st century.
Although over the past 60 years numerous studies have examined soil hydrologic processes, vegetation function, and micro-climate independently, investigating the feedbacks among these core areas has only recently become a research priority. Fundamental questions on vegetation’ effect on the hydrologic cycle remain unanswered: how is the vegetation water use linked to the water flows to groundwater and streams? to what extent does transpiration affect streamflow and groundwater? how does complex terrain, soil characteristics and land use influence the feedbacks between hydrology and ecology? Answering these questions is key to assess the influence of changing vegetation cover on hydrologic ecosystem services in agroforest environments.
Current soil-vegetation-atmosphere (SVAT) models assume that groundwater, streamflow and vegetation transpiration are all sourced and mediated by the same well mixed water reservoir—the soil (Romano et al., 2013). Indeed, a main tenant of forest and irrigation hydrology is that vegetation transpires water that would otherwise form streamflow and feed groundwater within a well-mixed subsurface reservoir. This vision has been recently and fundamentally challenged by a number of studies (Brooks et al., 2010; Penna et al., 2013; Good et al., 2015), which have shown evidence of eco-hydrological separation (the “two water world hypothesis”, McDonnell et al., 2014) —meaning that the soil water that supplies vegetation transpiration is isolated from the water that recharges groundwater and replenishes streamflow. Evaristo et al. (2015) provides widespread evidence of eco-hydrological separation across different biomes by using hydrogen and oxygen isotopic data. The lack of water exchange between soil pools questions previous conceptualizations and analyses of water-cycle processes (see Jasechko et al., 2013, for example), because it implies that methods for studying water partitioning that use measurements of isotope tracers in streams may be blind to the part of the soil-water balance that involves vegetation and soil evaporation.
These first studies delineate novel research lines because suggest a well compartmentalized eco-hydrological system, and indicate that vegetation uses, at least under some conditions, more tightly bound soil water than easily mobile soil water. Given that water moves through plants via gradients of water potential, the use of more tightly bound water, energetically more difficult to obtain, remains counterintuitive (Cassiani et al., 2015). Testing this ‘two water worlds (2WW) hypothesis’ represents therefore a grand challenge in hydrology (McDonnell, 2014; Good et al., 2015; Bowen, 2015) and would advance our understanding of relevant soil-vegetation-atmosphere feedbacks which shape hydrological fluxes and water availability under the impact of environmental changes.
Bowen G., 2015: Hydrology: The diversified economics of soil water. Nature, 525 (7567), 43-44.
Brooks R. et al., 2010: Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nature Geoscience, 3:100–104.
Cassiani G. et al., 2015: Monitoring and modelling of soil-plant interactions: The joint use of ERT, sap flow and eddy covariance data to characterize the volume of an orange tree root zone. Hydrology and Earth System Sciences, 19 (5), 2213-2225.
Evaristo J. et al., 2015: Global separation of plant transpiration from groundwater and streamflow. Nature, 525, 91-94.
Good S.P. et al., 2015: Hydrologic connectivity constrains partitioning of global terrestrial water fluxes. Science, 349 (6244), 175-177.
Jasechko, S., Sharp, Z.D., Gibson, J.J., Birks, S.J., Yi, Y., Fawcett, P.J., 2013: Terrestrial water fluxes dominated by transpiration. Nature, 496, 347-350.
McDonnell J. J., 2014. The two water worlds hypothesis: eco-hydrological separation of water between streams and trees? WIREs Water 2014.
Penna D. et al., 2013. Tracing the water sources of trees and streams: isotopic analysis in a small pre-alpine catchment. Proc. Env. Sci., 19, 106 - 112.
Romano N. et al., 2013: Parameterization of a bucket model for soil-vegetation-atmosphere modeling under seasonal climatic regimes. Hydrology and Earth System Sciences, 15, 3877-3893.
2 - Some of the methodology
The main goal of WATER-MIX is to advance the understanding of water partitioning and mixing within the Earth Critical Zone (CZ) by testing hypotheses of eco-hydrological separation of vegetation water use. For this, WATER-MIX will couple advanced isotopic, geophysical and micro-meteorological monitoring with detailed eco-hydrological models, and will particularly focus on the implications for water flow partitioning and water availability in the Mediterranean area. The investigation will sample across a transect of climatic, vegetation and elevation gradients, including both forested and agricultural ecosystems. Finally, the proposal will develop a framework to translate novel CZ knowledge into evidence to support water/land use policy and management decisions.
We define the following three key objectives for the project:
1) advancing the monitoring of water exchange and partitioning across the CZ by using integrated high-resolution isotopic, geophysical and hydro-meteorological measurements from point to catchment scale;
2) coupling the high-resolution CZ data set with eco-hydrological models at multiple scales to test hypotheses of i) eco-hydrological separation of vegetation water use , ii) residence time distribution and iii) energy partitioning across the CZ; 3) developing a framework to translate the new CZ-hydrology knowledge into evidence to support policy and management decisions concerning water and land use in forested and agricultural ecosystems.
2.2 The Critical Zone Observatories Network
The Project Critical Zone Observatories Network (CZN) includes five field sites (Fig. 1) which will provide a coherent access to different climatic, hydrological and ecological conditions which are representative of the Mediterranean and Alpine-Mediterranean environments. The CZN includes humid areas where vegetation water use and precipitation input are in phase, wet zones where seasonality of precipitation is low, and dry zones where water stress is high. Both forested and agricultural land use are represented in the CZN. Each CZ Observatory (CZO) involves co-located research to be conducted by interdisciplinary teams. The suite of measurements includes stable isotopic measurements, geophysical determination of soil water spatial distribution, land-atmosphere exchange of water, and linkages to the biosphere, surface and ground water systems. The CZOs are described in Section 3.
2.3 Structure of the work
To implement the project work, five WPs are defined and linked through a continuous exchange of information, with WP1 dedicated to the project management and dissemination of results. WP2 will develop a homogeneous protocol to integrate isotopic and geophysical observations with hydro-meteorological monitoring at various spatial scales to characterize water partitioning and balance across CZN. WP3 aims (i) at advancing isotope monitoring of vegetation and soil waters in order to help the identification of water pools and mixing processes and (ii) developing and implementing high-resolution, minimally invasive geophysical approaches to soil moisture content distributions, across the CZN. WP4 will couple the high-resolution CZN data set generated by WP2 and 3 with eco-hydrological models at multiple scales to test hypotheses of i) eco-hydrological separation of vegetation water use, ii) residence time distribution and iii) energy partitioning across the CZ. WP5 will develop a framework to translate the new CZ-hydrology knowledge into evidence to support policy and management decisions concerning water and land use in forested and agricultural ecosystems.