Comparison of rainfall reanalysis data to develop atmospheric scenarios during extreme El Niño episodes (observations and evaluations).

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Why studies of the water cycle are important?

Water cycle

Water is an integral part of life on this planet, and as we know, 70 % of the Earth’s surface is covered by oceans. It is always changing states between three phases (liquid, vapor, and ice) with these processes happening in the blink of an eye and over millions of years (Source: http://water.usgs.gov/edu/watercycle.html Accessed 10 Aug 2015). The movement of water in its different forms, continuous movement of water on, above and below the surface of the Earth, and the perpetual water phase changes are essential ingredients of the planetary water cycle (Fig. I.3). After the law of conservation of mass, the water mass on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables.
The water cycle is a closed circulation of the water in its three phases (solid, liquid and gaseous) inside the climate system. It is closed sequence of natural phenomena, in which the globe provides the atmosphere with water vapor that afterwards returns in liquid or solid phases to the globe (Leite and Peixoto, 1995). The natural phenomena that make up the water cycle are:
• the transfer of water from the globe to the atmosphere in its gaseous phase by the evaporation of liquid or solid water of the seas, lakes, river, etc. and the evapo-transpiration of plants at the surface;
• water transport in its gaseous and solid or liquid phases (clouds) by the general circulation of atmosphere;
• partial condensation of water vapor in small droplets and ice crystals that remain in suspension in the atmosphere, making the clouds;
• water transfer in its solid or liquid phases from atmosphere to the globe by hydro-meteor precipitation and deposition at the surface;
• runoff and storage at the surface, water infiltration at the subsurface and absorption by plan roots, the intensification of rivers on their way to the sea or lakes, and underground water storage (phreatic deposits).
And then this cycle starts again. The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence strongly the critical zone but also climate.

Why studies of the water cycle are important?

The physical processes governing the water and energy cycles are extremely compli-cated, involving scales ranging from the planetary to the microscopic one. Any alterations in atmospheric gaseous composition (water vapor, carbon dioxide, ozone, etc.), partic-ulates (desert dust, smoke, urban smog, etc.), or clouds (coverage and brightness) can disturb the radiative heat balance and result in chain reactions in the water cycle. It is very important for the climate community to not only closely monitor the regional and global water budget, but to also understand changes in frequency of occurrence and strength of individual weather events. This is especially true of extreme weather events, which have great societal and economic impacts. Whether we will have more or more intense tropical storms, mega-snow events, or dust-bowls in the near or far future climate is one of the key focus areas of climate research.
As we can see from figure I.1, the water cycle consists of an aerial branch, which transports water vapor, liquid and solid water of clouds by the general circulation of the atmosphere, and a terrestrial branch formed by the surface and subterranean run-off. The role of the terrestrial hydrosphere in the Earth’s climate system can be described by climate-related variables, such as radiation, precipitation, evapotranspiration, soil moisture, clouds, water vapor, surface water and run-off, etc. Measurements of these quantities are required to better understand the global climate and its variability, both spatially and temporally, and to help advance our understanding of the coupling between the terrestrial and atmospheric branches of the water cycle, and how this coupling may influence climate variability and predictability. In order to enhance the prediction of global water cycle variations, understanding of hydrological processes and its close linkage with the energy cycle is fundamental. Here, a number of key questions are addressed:
• How can we quantify the water cycle processes (storages and fluxes, as well as land-atmosphere, surface-groundwater, water-ecosystem and land-oceans inter-actions)?
• How can we assess the impacts of and the vulnerabilities to future climate change in water resources and what is the potential to adaptation in water resources management?
• What tools can be further developed/improved to predict the water cycle com-ponents (to measure, simulate and predict water cycle quantity and quality in space and time)?
Observations from space and in situ of water cycle components and their interactions with ecosystems, climate and human activities provide opportunities to address most of the above questions.
Figure I.3 – Diagram of the Water Cycle (v1.11). The water cycle shapes landscapes, transports minerals, and is essential to most life and ecosystem on the planet. It describes the pilgrimage of water as water molecules make their way from the Earth’s surface to the atmosphere, and back again. This gigantic system, powered by energy from the sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land (from Edhu Tal, 2016).

Why studies of the water cycle are important?

Factors Affecting Water Cycle

The water cycle is being affected by human activities, such as water resource exploitation, urbanization, and deforestation. All these controls the distribution of water and the cycles. Below we will consider three of these factors: human consumption, effects of global climate, land use changes, etc.

Human Consumption

We are becoming increasingly aware of our impacts on nature, but unfortunately many of the things we do have become so ingrained in our way of life that it is hard to change. Numerous human activities can impact on the water cycle: constructing dams, pumping water for farming, deforestation and the burning of fossil fuels. Every time humans inter-rupt the natural water cycle there will be an effect. We can summarize this interruption of water in two ways:
• Withdrawals: We take water out of the system to irrigate crops (Fig. I.4), to provide us with drinking water and to carry out many of our industrial processes (e.g. agri-food industry).
• Discharges: We add substances to the water, intentionally or not, as pollutants from increased agricultural needs and from increased economic activity (e.g. waste from sewage treatment plants).

Effects of Changing Global Climate

The hydrologic system is potentially very sensitive to climate changes. Changes in pre-cipitation affect the magnitude and timing of runoff and the frequency and intensity of floods and droughts. Changes in temperature results in changes in evapotranspiration, soil moisture, and infiltration.
The El Niño/Southern Oscillation (ENSO) phenomenon contributes seasonal-to-interannual variations in temperature and precipitation that complicate longer-term cli-mate change analysis in Pacific parts of the world. Climate anomalies (i.e., departures from the norm) associated with ENSO extremes vary both in magnitude and spatial dis-tribution. For example, the 1990 to 1995 persistent warm-phase of ENSO (which causes droughts and floods in many areas) was unusual in the context of the last 120 years (Tren-berth and Hoar, 1996). Although a relationship has not been found between increasing global temperatures and the occurrence of warm/cold-phase of ENSO events, this cli-mate phenomenon will certainly affect the content and nature of the global water cycle (Grimm and Tedeschi, 2009). This is especially true at the local and regional levels. All major climate change analyses to date have concluded that if temperature does increase as a result of increased greenhouse gases, the global mean water cycle will be enhanced and increased precipitation and soil moisture will occur, especially in high latitudes espe-cially during the winter (IPCC, 1996). All these changes are associated with identifiable physical mechanisms.

Land Use Changes

Land use, land cover can significantly influence the hydrologic balance and biogeochemical processes of watershed systems. These changes can alter interception, evapotranspira-tion, infiltration, soil moisture, water balance, and biogeochemical cycling of carbon, nitrogen, and other elements (Talib and Randhir, 2017). Changes in land cover patterns can directly impact energy and mass fluxes. For example, when large areas of forests are cleared, reduced transpiration results in reduced cloud formation, less rainfall, and in-creased drying. Changes in land cover can alter the reflectance of the Earth’s surface and induce local warming or cooling such as albedo (reflectivity) increases, surface temper-ature declines. Desertification can occur when overgrazing of savanna vegetation alters surface albedo and surface water budgets, and thus changes the regional circulation and precipitation patterns. Overgrazing can also increase the amount of suspended dust that, in turn, cause radiative cooling and a decline in precipitation (Salati and Vose, 1986).
Forests, and tropical forests in particular, play an important role in the global water cycle. The forest floors are for a large part responsible for evaporation in forests. Removal of forests will thus not only reduce the evaporation from the trees, but will also reduce the evaporation from the forest floor. The resulting local decrease of evaporation is very likely to have global consequences for rainfall, water resources and food security. Deforestation keeps water from returning to the atmosphere, resulting in changes in a number of characteristics of the watershed. Harvesting of timber or changing land use from farmland to housing developments can also increase runoff and cause the magnitude of flooding to be increased.
More development in flood plains and drainage basins can also damage the pattern of water flow by blocking the flow of water and increasing the width, depth, or velocity of flood waters. Ponds, lakes, reservoirs, and other sinks in the watershed also prevent or alter runoff from continuing downstream. Covering land surfaces with asphalt and other impervious surfaces, as evidenced by worldwide trends toward urbanization and urban sprawl, both increase runoff and inhibit replenishment of the ground water reservoirs, and thus affects the overall water cycle.
The water falling as precipitation (rain/snow/hail) in one region may have originated from a butterfly effect in a distant region, or that it may be recycled moisture that originated as evaporation within the region. Global wind patterns, topography, and land cover all play a role in moisture recycling patterns and the distribution of global water resources. Land use changes such as irrigation, dams, and deforestation can alter evaporation patterns in a region, potentially affecting water resources in distant regions (van der Ent et al., 2010).

Tools permitting the analysis of the water cycle

Water is perhaps the most important component of any ecosystem. Some water is stored deep in the earth. Surface water, on the other hand, is the source that sustains life on land. In many cases, water also structures the physical habitat of an ecosystem. Polar bears, for example, rely on ocean ice in order to successfully hunt and capture seals. Rivers, lakes, and other bodies of water divide environments into different habitats, effec-tively defining where some organisms can live and others cannot (Corenblit et al., 2007, 2011). What’s more, most of the life on Earth actually lives completely submerged in the waters of the oceans. Water is truly a powerful factor in all ecosystems (Fig. I.5). In a broad context, the sciences of meteorology and oceanography describe parts of a series of global physical processes involving water that are also major components of the science of hydrology. Geologists describe another part of the physical processes by ad-dressing groundwater movement within the planet’s subterranean features. Hydrologists are interested in obtaining measurable information and knowledge about the water cycle. Also important is the measurement of the amount of water involved in the transitional stages that occur as the water moves from one process within the cycle to other processes. Hydrology, therefore, is a broad science that utilizes information from a wide range of other sciences and integrates them to quantify the movement of water. The fundamental tools of hydrology are based in supporting scientific techniques that originated in math-ematics, physics, engineering, chemistry, geology, and biology. Hydrology, therefore, is one of the interdisciplinary sciences that is the basis for water resources development and water resources management.
Figure I.5 – The Coral Reef Temperature Anomaly Database (CoRTAD) is a collection of sea surface temperature (SST) map and related thermal stress metrics, developed specifically for coral reef ecosystem applications but relevant to other ecosystems as well. The CoRTAD Version 5 contains global, approximately 4 km resolution SST data on a weekly time scale from 1982 through 2012. The SST data are derived from the Advanced Very High-Resolution Radiometer (AVHRR) sensor.
An improved knowledge of the land surface hydrologic states and fluxes, and of their spatial and temporal variability across different scales, is an important goal in many hydrologic studies (Maurer et al., 2001; Roads et al., 2003). To this end, isolated tradi-tional approaches, such as in-situ observations (Ropelewski and Yarosh, 1998), ground observations driven modelling (Mitchell et al., 2004), or direct retrievals from remotely sensed variables (McCabe et al., 2005), become inadequate. To improve this situation, research is being carried out by a number of groups with the goal to utilize and combine as many data products as possible within an integrated framework in a manner to im-prove the estimation of the water cycle states and fluxes. This integration of data and models is referred to as data assimilations or data fusion (McLaughlin, 2002). Recently, remote sensing data are playing an increasingly important role in such efforts. The main reason is that remote sensing data provides large-scale, systematic land surface obser-vations consistently over the globe. Moreover, advancements in sensors, development of improved retrieval algorithms and tremendous increases in data distribution, storage and processing has greatly promoted the use of remote sensing data in hydrology (Pan et al., 2008). Water cycle is usually divided in two main compartments: the first one point out the oceanic water (§I.2.1) and the second one highlight the continental water (§I.2.2).

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Ocean Water cycle

From the major components of the water cycle (§I.1.1) have been briefly summarized we can turn to some of the modern tools used to study these processes. The empha-sis will be on the satellite-based sensors and accompanying models that the National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA) have launched since the mid-1970s.
Over the years, several NASA missions have studied the effects associated with the global water cycle including El Niño and La Niña, such as changes in sea surface tem-perature, cloud cover, ocean surface winds, and rainfall. These studies are augmented by data from operational satellites of NOAA. For examples:
• Initial efforts at mapping sea surface temperature and cloud cover were conducted using data from NASA’s Nimbus series of satellites. Also, this series included the first ocean color scanner (Coastal Zone Color Scanner), which provided the first estimates of phytoplankton productivity (chlorophyll concentrations) from space.
• The Advanced Very High Resolution Radiometer (AVHRR) instrument flown on NOAA’s TIROS-N weather satellite in 1978 and on the NOAA-6 satellite in 1979 greatly enhanced the accurate measurements of factors related to climate variability (e.g. Fig. I.5). Still further increases were added to the AVHRR instrument and on subsequent NOAA satellites.
• Sea Surface using radar altimeters – JASON-1, 2, 3; ENVISAT, ERS, SARAL, Sentinel 3 and TOPEX-POSEIDON (e.g. Fig. I.6): Satellite altimetry is a radar technique providing the topography of the Earth surface based on the measurement of the distance between the satellite and the surface derived from the two-way travel time of an electromagnetic wave emitted by the altimeter (or altimeter range) and the precise knowledge of the satellite orbit (Chelton et al., 2001; Biancamaria et al., 2017). For environmental applications, short repeat period is necessary, the recent generation of satellite altimeter tried to improve this parameter (Fig. I.7). The primary objectives of radar altimetry from satellites were to measure the marine geoid and monitor the dynamics topography of the sea surface (Vu et al. (2018), submitted) over the open ocean and the coastal areas (Fig. I.8).
Currently, there are many NASA missions that are simultaneously measuring a myriad of Earth’s water cycle variables; ocean/sea salinity or temperature (Fig. I.5) or tides (Fig. I.6) or height (Fig. I.7), Evaporation, Condensation, Precipitation, Groundwater Flow, Ice Accumulation, and Run-off. NASA’s water cycle research missions can be grouped into 3 major categories; Water Cycle, Energy Cycle, and Water and Energy Cycle Missions. NASA’s goal is to improve/nurture the following global measurements: precipitation (P), evaporation (E), P-E and the land hydrologic state, such as soil-water, freeze/thaw and snow. Therefore, an experimental global water and energy cycle observation system combining environmental satellites and potential new exploratory missions – i.e. advanced remote sensing systems for solid precipitation, soil moisture, and ground water storage – may be feasible. These proposed approaches are tantalizing, for knowledge of global fresh water availability under the effects of climate change is of increasing importance as the human population grows. Space measurements provide the only means of systematically observing the full Earth while maintaining the measurement accuracies needed to assess global variability.

Continental Water Cycle

The continental hydrologic cycle involves both land surface and its interaction in the critical zone in all phases via liquid and frozen precipitation, infiltration and recharge, surface run-off and snow-melt, stream/river flow, and evapotranspiration. These different transport mechanisms are interconnected and strongly affected by the land-atmosphere dynamics and surface heterogeneity in soil type, topography, and vegetation. Numerous international programs, such as:
• Prediction of Ungauged Basins (PUB) and Hydrology for the Environment, Life and Policy (HELP), which focus on the lack of rain gauge data within single wa-tersheds, have been instrumented in bringing worldwide attention to the lack of ground observations.
• The advent of the Earth Observing System (EOS) era in the 1990s, NASA, ESA, and JAXA have launched numerous space-borne sensors to study the various com-ponents of the terrestrial water cycle.
There are some uses/sensor/missions to estimate and monitor components of the water cycle that can be used to expand the knowledge in quantifying the spatial and temporal variations in the continental water cycle:
• Precipitation – Tropical Rainfall Measuring Mission (TRMM): Satellite estimates of precipitation are key to understanding the global water cycle with high tem-poral resolution. It can address particular challenges in analyzing precipitation over land, including understanding rain/no-rain classification, making retrievals over complex terrain, integration of model information (which is important in ar-eas where satellites have trouble making retrievals), and inter-calibration of all precipitation-relevant satellites.
Figure I.8 – Coastal performances between in situ (tide gauge) and altimeter based SSH in La Rochelle using different satellites: a) JASON-2 descending track 70; b) ERS2, descending track 0859; c) ENVISAT descending track 818, d) SARAL descending track 218. (modified from Vu et al. (2018)).

Tools permitting the analysis of the water cycle

• Evapotranspiration (ET) and sensible heat flux – Moderate Resolution Imaging Spectroradiometer (MODIS): is required at many different spatial and temporal scales in climate, weather, hydrology, and agricultural research and applications. Satellite remote sensing is viewed as one of the only technologies that can be used with land surface models to derive ET from field to global scales, particularly in regions with little or no ground resources available.
Figure I.9 – Snow map in the Pyrenees on 16 April 2017. The Theia snow product indicates the snow presence or absence on the land surface every fifth day if there is no cloud. This 20 m resolution product is derived from Sentinel-2 L2A images.
• Surface water – JASON-1, 2, 3 and TOPEX-POSEIDON: Water flux throughout the world’s rivers, lakes, and wetlands is one of the longest monitored components in terrestrial water balance science. The diversity of satellite sensors now avail-able provide invaluable complementary observations, such as water storage change estimates at the basin scale or spatially distributed geomorphological parameters.
• Snow: Water stored in snow cover is one of the main components of water balance in many parts of the world. The large spatial variability of snow characteristics, particularly in mountains, makes remote sensing a very important alternative to ground snow observations (Fig I.9).

Table of contents :

I INTRODUCTION 
I.1 Why studies of the water cycle are important?
I.1.1 Water cycle
I.1.2 Factors Affecting Water Cycle
I.2 Tools permitting the analysis of the water cycle
I.2.1 Ocean Water cycle
I.2.2 Continental Water Cycle
I.2.3 Existing tools for water cycle research
I.3 GNSS-R for water cycle studies
I.3.1 GNSS-R atmospheric studies
I.3.2 GNSS-R ocean studies
I.3.3 GNSS land/hydrology studies
I.4 Organization of the manuscript
II STATE OF THE ART OF GNSS-R 
II.1 What is GNSS
II.1.1 Principle of GNSS
II.1.2 Description and structure of the GPS system
II.1.3 Description and structure of the GALILIO system
II.1.4 Description and structure of the GLONASS system
II.1.5 Other constellations
II.1.6 The Positioning measurement
II.1.7 Augmentation systems
II.1.8 Perspective
II.2 Reflection of GNSS signals
II.2.1 Multipath
II.2.2 Specular and diffuse reflection
II.3 GNSS Reflectometry (GNSS-R)
II.3.1 GNSS-R Measurement Techniques
II.3.2 Opportunity of the signal reflectometry
II.3.3 Observable obtained from airborne platforms
II.3.4 Interference Pattern Technical – Reflectometer with single antenna
II.3.5 Platforms and constraints
II.4 Efficiency of GNSS-R
II.5 Conclusions
III GNSS-R for soil moisture estimation using Unwrapping SNR phase for sandy soil 101
III.1 Introduction
III.2 Objective
III.3 GNSS R – SNR data for detecting soil moisture
III.3.1 Observation Sites
III.3.2 Soil moisture retrieval
III.3.3 Relation between soil moisture and vegetation height retrieval
III.4 Article – Monitoring of soil moisture dynamics in sandy areas using the unwrapping phase of GNSS-R
IV GNSS-R for detection of extreme hydrological events: Red River Delta and Mekong Delta (Vietnam) 
IV.1 Introduction
IV.2 Methodology
IV.2.1 SNR-Least Square Method for the estimation of the continental water level
IV.3 Mekong Delta experiment (Vietnam)
IV.3.1 Presentation of the measurement site and experimental conditions
IV.3.2 Parameters for SNR data analysing
IV.3.3 Comparison between the height derived from GNSS-R and in-situ gauge records
IV.4 Red River Delta experiment (Vietnam)
IV.4.1 Presentation of the study area and datasets available
IV.4.2 Parameters for SNR data analysing
IV.4.3 Results
IV.5 Conclusion
V Conclusion and perspectives 
V.1 Conclusion
V.2 Perspectives
Bibliography

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