Thriving on Our Changing Planet A Decadal Strategy for Earth Observation from Space (2018) / Chapter Skim
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6 Global Hydrological Cycles and Water Resources
6 Global Hydrological Cycles and Water Resources
Pages 225-295
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From page 225... ...
-- have provided important measurements of shortwave and longwave radiation, snow and glacier extent and change, soil moisture, atmospheric water vapor, clouds, precipitation, terrestrial vegetation and oceanic chlorophyll, and water storage in the subsurface, among many others. Visual, infrared, and lightning imagery from Geostationary Operational Environmental Satellites (GOES)
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will enable detection of surface disturbance by identifying subtle changes in surface elevation. As a part of the Decadal Survey for Earth Science and Applications from Space, the Panel on Global Hydrological Cycles and Water Resources (Hydrology, or "H")
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Methods for monitoring and modeling of the water cycle, and their application to societal goals, cover the wide range of needs for a comprehensive understanding of the hydrologic cycle as they relate to freshwater availability, water quality for human health and ecosystem services, and prediction of extremes and hazards. These extend from the accurate quantification of water and energy fluxes at the river basin scale, to accurate snow water equivalent (SWE)
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H-2c. Quantify how changes in land use, land cover, and water use related to agricultural activities, food production, and forest management affect water quality and especially groundwater recharge, threatening sustainability of future water supplies.
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The study of our hydrologic cycle, and how it changes over time, is critical to understanding and quantifying freshwater availability, water quality and ecosystem health, and anticipating and managing risks due to extremes. Remotely sensed data have permitted the scientific community to develop broad new understandings of the water cycle at scales from small basins to continents and the entire Earth, and to advance socially important applications.
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. As Figure 6.2 shows, all components of the water cycle are linked at scales ranging from global to small basins, impacting and being impacted by human activities such as water withdrawals for agriculture and infrastructure development such as dams (Dalin et al., 2017)
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Benefits of Prior Efforts The previous decadal survey (NRC, 2007a) , emphasized the need for high-quality global estimates of precipitation, soil moisture, and snow-water equivalent.
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. • he Soil Moisture Active-Passive (SMAP)
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and a developing suite of precipitation sensors rely on measurements from multiple satellites and collaborations with international partners, mainly space agencies and centers in Europe, Japan, and India to measure systematically key geophysical variables including shortwave radiation, atmospheric composition, clouds, precipitation, soil moisture, terrestrial vegetation and oceanic chlorophyll, water storage in the subsurface, and land subsidence, among many others, thus effectively establishing a de facto Earth Observing System. Such integrated observations show interrelatedness and feedbacks among seemingly removed processes and states, such as atmospheric composition and evapotranspiration, linking atmospheric pollution to clouds and surface temperature and water availability, and linking, in turn, public and environmental health to irrigation needs for food production and energy security.
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H-1: Coupling the Water and Energy Cycles Question H-1. How is the water cycle changing?
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contains the largest deposit of ice and snow outside the polar regions; here, shrinking glaciers provide evidence of climate change in one of the world's iconic regions, and the region plays a critical role in controlling the land-surface energy balance, and downstream irrigation and freshwater availability in several densely populated river basins (Kehrwald et al., 2008)
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Thus, the key to estimating evapotranspiration lies in measuring, or modeling, the variables and parameters that determine other terms of the energy balance equation -- solar and longwave radiation, albedo, surface temperature, air temperature and atmospheric water vapor pressure in the boundary layer, and wind. Surface soil moisture influences the boundary layer cloud development through the latent heat flux associated with evapotranspiration, which in turn regulates surface temperature and thus the sensible heat flux and emitted longwave radiation, thereby affecting net surface radiation and available energy (Betts, 2004; Ek and Holtslag, 2004; Findell and Eltahir, 2003)
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These processes are explicitly included in climate models, where the surface water and energy cycles are closed (fluxes in balance) by mathematical design at model-resolved scales that are unfortunately much coarser than the governing process scales, which are therefore not appropriately represented (i.e., parameterized)
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2. o what extent have the water cycle components and their variability changed, and have these T resulted in changes to extreme events (floods and droughts)
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worldwide at convective and orographic scales suitable to capture flash floods and beyond as well as processes at longer and larger spatial scales. The Global Hydrological Cycles and Water Resources Panel assigned highest priorities to developing an Integrated Earth System analysis, which would integrate models and observations, and to measuring rainfall and snowfall and accumulated snow on the ground, which are key constraints and inputs into that analysis.
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to ongoing measurements of precipitation, soil moisture, sea ice, and other variables sensitive to water fluxes at the land- and ocean-atmosphere interfaces, including at short time scales, as high- revisit passive microwave spectrometry from space becomes available (e.g., TROPICS mission)
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Albedo, areal extent (snow-covered area, SCA) , and snow water equivalent (SWE)
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. Although significant progress has been achieved to estimate snow water equivalent from passive microwave observations (Kelly and Chang, 2003; Li et al., 2015; Shi et al., 2016)
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Coupled with the problem of knowing the total quantity and spatial distribution of the snow accumulation are measuring and predicting its rate of melt, relating the rate of melt to environmental drivers, and the consequences of the rate and distribution of melt for water resources, glaciers, and ecosystems. The main drivers, absorption of solar and longwave radiation, vary with the solar geometry, atmospheric scattering and absorption, and illumination variability caused by topography (Marks et al., 1992; Marks and Dozier, 1992)
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Specifically, the evapotranspiration flux and the terrestrial surface water budget have changed dramatically over the historical record as a result of human alteration of the landscape. Because sensible and latent heat fluxes are fundamentally coupled by thermodynamics, these changes have already had significant impact on the terrestrial surface energy budget, including surface temperature and outgoing longwave radiation.
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and root zone soil moisture uptake, and in turn root zone soil moisture availability modulates stomatal conductance, and consequently evapotranspiration (ET) , and photosynthesis, and consequently gross primary productivity (GPP)
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SMAP measures soil moisture in the top 5-10 cm and has enabled understanding of links between precipitation, surface soil moisture, and energy fluxes at very coarse spatial scales (10 s km)
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Specifically, the impact of conversion of native land to agriculture alters the terrestrial water cycle in both quantity and quality. These changes in the land use and land cover affect infiltration, surface runoff, recharge to the groundwater, water quality, sediment loss, and surface albedo, as well as affecting the temporal dynamics of all of these processes.
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Concomitant with the agricultural water quantity problems are ongoing degradation in groundwater quality owing to salt, nitrate, and other contaminants inherent to agricultural practices. Worldwide, increasing difficulty in sustainably managing water quantity and quality, whether in the subsurface or on the surface, remains a major challenge to soil conservation, food production, and the future of human civilization.
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While water availability, environmental and ecosystem health, and social and economic well-being are directly affected by water quantity and quality, current Earth observation missions are not fully equipped to measure the quality of inland water bodies such as lakes, rivers, reservoirs, and estuaries, at an appropriate temporal and spatial scale, and enabling technology is still lacking. In addition, there are many water quality indicators that vary independently while having a combined effect on the remote sensing signals (Ampe et al., 2015)
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In many parts of the world, complete transformation of land cover including deforestation, extensive row crop agriculture, and urbanization are affecting the eco-hydrology at local, regional, and continental scales. Change of fluxes and storages, and the transport and residence times of water in the terrestrial part of the landscape, in the streams, and in the subsurface affect bio-geochemical processes and impact water quality and stream biology.
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If advances in lidar technology can continue apace so that within 10 to 15 years, spaceborne platforms can provide the 20 to 30 return points per square meter that airborne platforms currently provide, a rich set of attributes on vegetation structure and productivity could be derived to constrain and improve evapotranspiration estimates. The additional benefits of these data to hydrology include improved topography for flow path and drainage derivations, surface water levels, snow depths, and near shore bathometry.
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Several applications of Earth observations exist for improved hazard mitigation. In mountainous regions, frequent measurements of near-surface soil moisture and of snow water equivalent (SWE)
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; though not all were rainfall induced, the great majority were. Flash floods and shallow, rainfall-induced landslides are caused by high-intensity, high-volume precipitation events coupled with the proper hydrological scenario, which is defined by current soil moisture; the slope, shape, and soil types of the basin; the impervious region in the basin; and the built drainage structures of the basin.
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Flash flood points correspond to river gauge locations; polygons represent areas impacted by the flash flood, often coinciding with county boundaries. Note that flash floods are largely underreported, as many occur in ungauged basins, which frequently are not populated.
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The index integrates slope land cover change, geology, and road presence, and operational nowcasts of landslide potential are made by checking whether 1-day, 3-day, and 7-day precipitation intensity thresholds and accumulations are met. Further details and up-to-date forecasts are available at https://pmm.nasa.gov/precip-apps.
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The interactions between the solid Earth and land cover and surface dynamics -- for example, earthquake triggering of ice avalanches and landslides, landslide blocking of rivers, and consequent landslide-dammed lake outburst floods -- can be strongly influenced by the preceding history of the thermal state and retreat of glaciers, or by the seasonal state of shallow groundwater saturation of soils and snow melting (Kargel et al., 2016)
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. In situ soil moisture is one of the least observed aspects of the hydrologic cycle in terms of long-term, large-scale measurements.
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Improved predictions of inflows into water supply reservoirs are a key requirement for early warning and planning of such hydrological droughts. For agricultural drought, soil moisture is the monitoring variable.
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. Key needs in space-based estimates for land cover/land use change include higher resolution soil moisture, including at scales for better estimates of plant water availability for evapotranspiration (i.e., less than 1 km and at a daily scale)
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For the hydrologic cycle, the concern mainly addresses the disposition of net radiation at the surface. At spatial scales of 100 km or so, and especially when averaged over time and space, the surface and top-of-atmosphere estimates of solar and longwave radiation from the Cloud-Earth Radiant Energy System (CERES)
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the surface layer, specifically topography sensors in the Program of Record: evapotranspiration, which is not CERES, MODIS, VIIRS, GOES-16, directly measured but instead Himawari, MSG inferred from the energy fluxes to Albedo (vegetation and soil, separately) Imaging spectrometer get the latent heat flux Soil moisture in root zone L-band and P-band radiometer and radar Diurnal cycle of surface temperature Thermal infrared, in 4 µm and 11 µm (vegetation, soil, snow)
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H-4b Flash floods Same as H-1b, H-1c, H-4a H-4c Drought monitoring Same as H-1b, H-1c, H-2c H-4d Linkage between land modification Same as H-2a, H-2b, H-4a, H-4b, H-4c and hazards NOTE: New measurements are indicated in italics. Broadband albedo of a surface is the convolution of the spectral albedo with the spectral distribution of solar radiation at the surface.
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An active-passive L-/P-band system would also support continuity of surface soil moisture estimation using the NASA SMAP and ESA SMOS missions. Soil moisture, a state variable of the land branch of the water cycle, has temporal variations ranging from daily to interannual.
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Surface temperature and vegetation observations are available at a finer spatial resolution than soil moisture, and this along with the relation between soil moisture and diurnal change in surface temperature can be used to downscale the coarser spatial resolution soil moisture (Fang et al., 2013)
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Accurate estimation of precipitation from space is thus of paramount importance for improving weather and climate prediction models, for closing water budgets at the catchment scale, for providing global coverage of this most critical component of the water cycle, and for early prediction of severe storms. The Tropical Rainfall Measuring Mission (TRMM)
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The challenge of providing accurate precipitation estimates everywhere and at spatial and temporal resolutions needed for accurate and timely prediction of extreme weather and floods remains a critical issue in global water and food security and in global health. From GPM this challenge will be met in the coming decade by combining products from several sensors, and learning from the coincidental active and passive sensors to improve physics-based and data-learning retrieval algorithms.
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. The radar system should further be coupled with a high-frequency radiometer, not unlike the current ATMS sensor, but with increased spatial resolutions to match the radar FOVs.
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Snow and Glaciers The difficulties in measuring snowfall and mixed-phase precipitation are partly compensated by the fact that snow lies on the ground for a while before it melts or sublimates. Therefore, a complementary strategy is to measure snow depth and density and thereby snow water equivalent (SWE = snow depth × snow density)
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uses lidar to measure snow depth by comparing elevations measured weekly or biweekly throughout the snow season with the same topographic data acquired when free of snow in the summer. Snow water equivalent is estimated by multiplying the depths (at spatial resolution of ~3 m)
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. Wind transport can be identified by measurements of snow water equivalent.
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. This disparity in scales of the GRACE measurements and the hydrologic system or problem means that measurements from GRACE in even large groundwater basins such as California's Central Valley include not only the changes in groundwater storage in the major sedimentary aquifers, but also the changes in snow in the adjacent mountain range, soil moisture, and fractured-rock groundwater, which all must be measured and modeled sufficiently to separate them from the important major aquifer storage changes.
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Besides irrigation water management, the other forcing that will affect recharge is climate change. Warming will tend to increase potential evapotranspiration, decreasing recharge; but on the other hand, climate-induced changes in vegetation due to reduced soil moisture and deeper groundwater levels may result in less actual evapotranspiration.
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Collectively, this approach will become increasingly essential for local and regional groundwater management. Water Quality One of the essential but overlooked elements of regional water availability and sustainable water resources management is water quality.
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. In addition, considering the advancement of drones and other imaging technologies, there are possibilities for regions with extensive water quality challenges to rely on these technologies to collect water quality data at a finer temporal and spatial scale.
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RESULTING SOCIETAL BENEFIT The crucial question for our interactions with water is: How can we protect ecosystems and better manage and predict water availability and quality for future generations, given changes to the water cycle caused by human activities and climate trends? The problem of sustainable water resources management is itself a grand research challenge not only because prediction of future forcings is challenging but also because real-time measurements of the state of the hydrologic systems, including the essential stores and fluxes of surface water and groundwater, are commonly lacking (NRC, 2012, 1991)
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InSAR has applications in other areas of Earth science, and if such a technology is selected for launch in the 2024-2034 time frame, then exploring its use in estimating groundwater balance through measurement of elastic subsidence is a fruitful research area for NASA and its partners. Remote Sensing of Snow Water Equivalent Figure 6.12 shows an example of how remotely sensed information increased confidence in water management.
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has measured spatially distributed snow depth weekly in the springtime from near-peak accumulation through the end of the snowmelt season with a scanning lidar. Snow water equivalent is estimated by multiplying these values with snow density, which is determined from measurements at snow sensors in the basin and interpolated in combination with a snowmelt model.
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Another study used a conceptual framework and multiple environmental models to assess the socioeconomic impact of Landsat-based agricultural mapping and groundwater quality (Forney et al., 2012)
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. Predicting drought onset and end depends on currently uncertain long-range precipitation predictions and good soil moisture measurements (Wood et al., 2015)
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Even slight improvements in our ability to manage our water resources to mitigate drought impacts can potentially save many millions of dollars. Conceptually, we understand the water cycle, but our quantitative understanding of its components is limited to some fluxes -- streamflow and precipitation, for example -- and surface water stores (NRC, 1991, 2002, 2008)
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2016. Active and passive microwave remote sensing synergy for soil moisture estimation.
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2016. Validating reconstruction of snow water equivalent in California's Sierra Nevada using measurements from the NASA Airborne Snow Observatory.
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2010. Evaluating the utility of remotely sensed soil moisture retrievals for operational agricultural drought monitoring.
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2012. Evaluation of SNODAS snow depth and snow water equivalent estimates for the Colorado Rocky Mountains, USA.
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2013. Passive microwave soil moisture downscaling using vegetation index and skin surface temperature.
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Geophysical Research Letters 40(17)
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2004. CMORPH: A method that produces global precipitation estimates from passive microwave and infrared data at high spatial and temporal resolution.
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2017. Surface soil moisture retrieval using the L-band synthetic aperture radar onboard the Soil Moisture Active-Passive satellite and evaluation at core validation sites.
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2013. Small scale spatial variability of snow density and depth over complex alpine terrain: Implications for estimating snow water equivalent.
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2013. Subgrid variability of snow water equivalent at operational snow stations in the western USA.
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2016. The Airborne Snow Ob servatory: Fusion of scanning lidar, imaging spectrometer, and physically-based modeling for mapping snow water equivalent and snow albedo.
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2016. Spatial estimates of snow water equivalent from reconstruction.
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2016. Review of snow water equivalent microwave remote sensing.
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2013. The effect of spatial variability on the sensitivity of passive microwave measurements to snow water equivalent.
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2016. The contribution of reservoirs to global land surface water storage variations.
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