At sunrise in Bundelkhand, Northern India, farmer Ramesh Patel walks across a cracked wheat field, where the delayed and weak monsoon has left his crops wilting and wells running dry. Over the past five years, wells have sunk several meters, and groundwater levels in the region have fallen steadily for more than two decades (CGWB 2021; Niranjannaik et al. 2022). He is not alone: farmers, water managers, industries, and hydropower operators around the world face the same urgent question—how much water do we really have?  

UNESCO estimates that 2.2 billion people currently experience water shortages, while agriculture accounts for approximately 72 percent of global freshwater withdrawals, and demand is projected to rise by 55 percent by 2050 (UNESCO 2024). As scarcity intensifies, conflicts among agricultural, urban, and industrial users are growing. International frameworks such as Sustainable Development Goal (SDG) 6 which calls for ensuring availability and sustainable management of water and sanitation for all and the Paris Agreement, which emphasizes water resilience as part of climate adaptation, both underscore the importance of evidence-based water governance (United Nations 2015; UNFCCC 2015).

This is where water accounting and auditing become essential. They help answer not only “How much water do we have?” but also “Where is it going?”, “Who uses it?”, and “Is it being used sustainably?” Advances in space-based technologies including the Gravity Recovery and Climate Experiment (GRACE), the Surface Water and Ocean Topography (SWOT) mission, and the Soil Moisture Active Passive (SMAP) satellite are revolutionizing these practices by tracking groundwater depletion, surface water levels, and soil moisture in near real time . Systems such as Open Evapotranspiration (OpenET) and the Food and Agriculture Organization’s Water Productivity through Open-access of remotely sensed derived data (WaPOR) platform are now transforming satellite data into actionable maps for irrigation planning and water productivity (NASA 2023). Together, these “eyes in the sky” provide transparent and cost-effective monitoring, even in remote or transboundary regions, enabling policymakers to design fairer water-sharing rules, guide conservation, and ensure that every drop is measured and managed wisely (UNESCO 2023). A global shift toward evidence-based water governance is now within reach and space technology lies at its core. 

Water accounting and water auditing

What is water accounting and auditing?

Water challenges are intensifying due to climate variability, rapid population growth, and competing demands from agriculture, cities, energy sectors, and ecosystems. To manage water effectively and equitably, countries and river basin authorities are increasingly turning to water accounting and water auditing—two complementary tools that enhance transparency and accountability in water governance, much like financial accounting does for money.

Water accounting is the systematic process of identifying, quantifying, and reporting water flows, stocks, and uses within a defined spatial and temporal boundary (Molden 1997; FAO 2016). It answers fundamental questions on the quantity, sources and uses of this valuable resource. By providing a structured and comparable framework, water accounting enables planners to identify inefficiencies, evaluate trade-offs, and support sustainable allocation.

Water auditing, in contrast, evaluates how water is managed by assessing losses, allocation efficiency, governance performance, and compliance with established water plans or regulations (Global Water Partnership 2012; FAO 2012). It promotes transparency by ensuring that water data is reproducible, verifiable, and subject to independent review. Institutional integrity is central to this process: audits must be conducted or certified by qualified professionals to maintain credibility. For example, in the U.S. state of Georgia, the Qualified Water Loss Auditor (QWLA) certification administered by the Georgia Environmental Protection Division ensures that auditors possess the technical expertise required to validate and certify water audits accurately (Georgia EPD 2020). Addressing uncertainty is equally vital, as errors stemming from measurement inaccuracies, data handling, or human estimation can lead to misinterpretation (WRF 2016). A comprehensive water audit checklist (Figure 1) typically includes assessing system input volumes, identifying authorized consumption, and quantifying both real and apparent losses — steps that together support evidence-based water management and policy development (Water Efficiency Alliance 2021). 

While water accounting is about measurement, water auditing is about evaluation and accountability. Water accounting and auditing follow a structured and iterative framework that integrates technical assessments with stakeholder engagement. Figure 2 shows the overall approach to water accounting and auditing illustrating the interrelationship between water accounting, water auditing, and stakeholder engagement activities.

Together, these tools support Integrated Water Resources Management (IWRM) by improving transparency, reducing conflicts, and guiding equitable allocation. 

The water budget concept

The water budget concept is central to water accounting and water auditing. This concept It helps in understanding the dynamics of water availability and usage within a system. Figure 3 illustrates the total water budget concept, showing that all major fluxes, including precipitation, streamflow, groundwater recharge, and evapotranspiration, are tracked across the land system, surface water system, and groundwater system to ensure a complete accounting of water inflows and outflows.  

At the core of all water accounting systems is the water balance equation (Healy and Scanlon 2010), also known as the water budget:

where P is precipitation, is water entering the system, ET is evapotranspiration (including soil, plant, and surface water losses), ΔS is the change in storage, and is water leaving the system. This equation embodies the principle of mass conservation, essentially stating that water entering a system must equal water leaving it plus any changes in storage.

This equation reflects a simple truth: water in = water out ± storage change (Healy and Scanlon 2010). Whether applied to a farm, aquifer, reservoir, river basin, or nation, every water account begins with this balance. 

Frameworks for water accounting

Several international frameworks formalize water accounting to support policy and planning. The System of Environmental-Economic Accounting (SEEA)-Water framework by the United Nations Statistics Division (UNSD)(Replace with official acronym) links hydrology with national economic statistics for policy reporting especially SDG 6.4 on water-use efficiency (UNSD 2012). Water Accounting Plus (WA+), led by Food and Agriculture Organization (FAO) and United Nations Educational, Scientific and Cultural Organization – Institute for Water Education (UNESCO-IHE), produces basin-level water accounts using remote sensing and GIS making it especially valuable in data-scarce regions (Karimi et al. 2013; Bastiaanssen et al. 2015). Unlike SEEA-Water, WA+ explicitly separates beneficial water consumption (e.g., crop evapotranspiration) from non-beneficial evaporation (e.g., evaporation from canals). Additionally, the concept of basin-level water audits, as advanced by the FAO, provide diagnostic assessments of water governance, resource availability and use. These audits help guide sustainable water management decisions by integrating technical, institutional and political dimensions. water governance and scarcity, helping guide sustainable water management decisions.

“Sustainability is key. Water accounting, which integrates satellite data, modelling, and ground-based measurements, can provide a better understanding of water availability, how and when to use it, and how it changes over time. Effective planning, especially for groundwater, is critical for sustainable water management.”   
— Dr. Claudia Sadoff, Director General, International Water Management Institute (IWMI, 2019)

To make water accounting and auditing truly effective at scale, we now turn to the space technologies that provide the data backbone for these methods. 

Space-based technologies for water accounting and auditing

Space-based technologies have transformed water accounting by providing accurate, timely data on groundwater, surface water, soil moisture, precipitation, and evapotranspiration.  Table 1 provides an overview of relevant datasets. These measurements by satellites complement ground observations, and are thus enabling evidence-based water management, efficient allocation, and drought preparedness. 

With a foundation in satellite capabilities established, the next step is to explore how these datasets are transformed into actionable water accounts and audits.

Methods: From satellites to accounts

Reliable water accounting and auditing frameworks use satellite-derived datasets combined with hydrological models and field validation to create water balance assessments and water use ledgers at basin, national, or irrigation-district scales. The workflow progresses from raw space-based observations to decision-ready water accounts through a transparent sequence of steps.  

Step 1: Satellite data acquisition

Earth observation missions provide continuous monitoring of key hydrological variables relevant to water accounting as shown in Table 2. For example, evapotranspiration (ET) and biomass production are retrieved from MODIS and Landsat datasets, while surface water extent is identified using Sentinel-1 SAR and Sentinel-2 optical imagery (Bastiaanssen et al. 2015). FAO’s Water Productivity through Open Access Remotely-Sensed Derived Data (WaPOR) portal delivers 10 m to 250 m resolution ET, biomass, and land use layers across Africa and the Near East, making it a practical operational data services supporting water accounting (FAO 2020). As shown in Figure 4, the WaPOR system translates remote sensing data into actionable insights for improving agricultural water efficiency. 

     
Step 2: Pre-processing and data harmonization

During the pre-processing, raw satellite data are corrected for atmospheric effects, resampled to common spatial grids, and temporally aligned using compositing methods such as maximum value composite (MVC) or median compositing, which merge multiple observations from a given period to reduce noise and data gaps caused by clouds and sensor errors. Cloud masking and noise reduction are applied to remove contamination from clouds, haze, shadows, and sensor noise, ensuring that only reliable surface reflectance data are used for further analysis (Senay et al. 2016).  

Input datasets are harmonized before hydrological balance calculations to ensure consistency between precipitation (P), evapotranspiration (ET), runoff (Q), and storage change (ΔS) components.

Step 3: Model fusion and partitioning (WA+ workflow)

After satellite data are acquired and pre-processed, water accounting frameworks typically combine multiple datasets with hydrological models to partition water flows and quantify their use by sector, ecosystem, or service type. This process, often referred to as model fusion and partitioning, integrates information on precipitation, evapotranspiration, runoff, and storage change from remote sensing with field-based or modeled hydrology to produce consistent, spatially explicit water accounts. Numerous modeling frameworks exist to perform this integration, including custom basin-scale models, regionally adapted hydrological simulators, and operational water accounting systems.

For demonstration purposes, we focus on the Water Accounting Plus (WA+) framework (Karimi et al., 2013). WA+ is widely used in conjunction with FAO’s WaPOR dataset because it provides a transparent, operational workflow that distinguishes between beneficial and non-beneficial water uses, partitions flows by land-use type, and supports decision-making at both, basin and managed-unit scales. While other frameworks could be applied, WA+ was chosen here due to its clear methodology, public accessibility, and strong precedent in water accounting literature, making it an ideal example for illustrating the transition from satellite observations to actionable water accounts.

The Water Accounting Plus (WA+) framework integrates remote sensing products with hydrological modelling to partition water flows by land use and service type (Karimi et al., 2013). Using WaPOR-derived ET and biomass data, WA+ distinguishes beneficial uses (crop production, ecosystem services) from non-beneficial ones (evaporation from bare soil or water surfaces).

At the basin scale, the total water balance is expressed as:

where: 
= precipitation, 
and = surface + groundwater inflows and outflows, 
ET= actual evapotranspiration (depletion), and 
= change in storage (surface, groundwater, snow, and soil moisture).

For managed water-use units (e.g., irrigation districts) where withdrawals and returns are explicit, the equation becomes:

where W= withdrawals, R= return flow, = ET from precipitation, and  = incremental ET attributable to withdrawals (Karimi & Bastiaanssen, 2015).

• Water productivity (kg/m³) (Productivity Sheet- Profit) 

• Renewable supply and environmental flows (Withdrawal sheets – Cashbook) 

This is visualised in Figure 5 by the Sheets (which contain the Water Ledger) and the resulting graphs and maps (which are the annual and spatial accounts), shown at the bottom and right. 

 Step 5: Handling uncertainty and validation

Due to sensor limitations and atmospheric interference, uncertainty corrections are required for satellite retrievals; validation targets the satellite-derived retrievals and the hydrological/data-assimilation models that generate WA+ components (e.g., ET, precipitation, surface extent/height, and TWS), using observations from river gauges, weather stations, soil-moisture probes, lysimeter and eddy-covariance ET measurements, groundwater wells, and lake/reservoir gauges (McCabe et al., 2017). The Remote Sensing Correction for Groundwater (RECOG) method further improves WA+ outputs by adjusting satellite water balance estimates using GRACE-based storage anomalies (van Dijk et al., 2018; Deggim et al., 2021). This step ensures robust and transparent water accounts that are aligned with observed hydrology.

To see these methods in action, we will now examine a few real-world applications where space-based water accounting has improved decision-making across diverse regions.

Global applications of satellite-based water accounting and auditing

Satellite technology is transforming global water management by revealing hidden patterns in water use and scarcity. In California’s Central Valley, the GRACE satellite exposed critical groundwater depletion averaging 2.41 km³ per year during 2003-2021; and peaking at 8.58 km³ per year during the 2019–2021 megadrought. This important observation lead to new groundwater regulations under the Sustainable Groundwater Management Act (SGMA) (Liu et al., 2022). As shown in Figure 6, the GRACE mission also revealed significant groundwater changes across the U.S. Water Accounting Plus (WA+) has been used in major basins like the Indus and Vietnam’s Ca River to quantify water flows and consumption using remote sensing, improving irrigation planning and national water policy decisions (Karimi, Bastiaanssen, and Fenn 2013; Bastiaanssen, Ha, and Fenn 2015). The Surface Water and Ocean Topography (SWOT) mission greatly enhances monitoring of lakes and reservoirs across diverse settings, from large inland systems like the Great Lakes and African rift lakes to cascade and run-of-river reservoirs in the Mekong and Amazon basins by providing high-resolution, spatially continuous maps of water levels and extents that reveal seasonal and event-scale dynamics previously undetectable from single-track altimeters (Biancamaria, Lettenmaier, & Pavelsky, 2016). Complementing this, GRACE and GRACE-FO capture basin-scale terrestrial water-storage variations critical for managing ecosystems and supporting river-dependent communities where groundwater–surface water exchanges and floodplain storage sustain livelihoods and navigation. Illustrative examples include floodplain populations and ports along the Amazon mainstem (e.g., Manaus, Iquitos, Belém), where millions of people rely on floodplain fisheries, agriculture, and navigation (Renó, Novo, & Escada, 2016); communities shaped by the Mekong flood pulse, where seasonal inundation supports fisheries and paddy agriculture essential to over 60 million people (Tran et al., 2024); and riparian settlements across the Ganges basin, where more than 500 million people depend on river water for food production and livelihoods (Singh et al., 2016). 

Building on these successes, the next section outlines how countries and water managers can begin implementing space-enabled water accounting systems.

Implementing space-enabled water accounting

Implementing space-enabled water accounting involves translating satellite observations into actionable water management information through a structured, phased approach. This roadmap addresses both, technical and institutional challenges, ensuring scalable, transparent, and sustainable water governance (FAO, 2016; Bastiaanssen et al., 2015; Sheffield et al., 2018; UNESCO, 2021).  

By following a stepwise framework, water managers can gradually integrate satellite-based monitoring into planning and decision-making, build local capacity, ensure independent verification, and ultimately scale water accounts from local to regional or national levels. The phased approach is summarized in Table 3.

Step 1: Pilot Testing 

Pilot testing focuses on selecting small, representative basins to implement the WA+ workflow. Using satellites such as GRACE/GRACE-FO for groundwater, SWOT for surface water, and Sentinel series for evapotranspiration and soil moisture, pilot tests provide initial insights into water stocks, flows, and deficits (Bastiaanssen et al., 2015). This low-risk stage validates data integration and workflow functionality, though limited spatial resolution and initial investment costs may restrict capturing full variability.

Step 2: Open Data Adoption

Open data adoption promotes the use of publicly available satellite datasets, open-source processing tools, and transparent workflows (FAO, 2016). Platforms such as FAO WA+, NASA Earthdata, ESA Copernicus, MODIS, and Landsat enable reproducible and cost-effective water accounts. Open adoption facilitates inter-agency collaboration and scalability, although data gaps, cloud cover, and the need for local calibration remain challenges.

Step 3: Capacity Building 

Capacity building ensures that water managers, policymakers, and stakeholders gain the knowledge and skills to use water accounting outputs effectively (UNESCO, 2021). Workshops, online courses, and research collaborations strengthen local technical expertise, enhance decision-making, and foster ownership of water management processes. Sustaining training programs and mitigating staff turnover are key challenges in this step.

Step 4: Operationalisation 

Operationalisation integrates water accounts into routine water planning and management. By combining WA+ modeling with satellite and in-situ measurements, managers can monitor water stocks, flows, and withdrawals in near real-time (Sheffield et al., 2018). This step enables evidence-based planning, proactive drought management, and optimized water allocation, though coordination across agencies and resistance to change may hinder implementation.

Step 5: Independent Audits 

Independent audits introduce verification and quality control mechanisms to ensure accuracy and credibility. Methods include RECOG corrections for GRACE and combined satellite-field validation (FAO, 2016). Independent audits enhance transparency, build stakeholder trust, and validate sustainable water use, although political sensitivity, funding constraints, and limited data access can pose challenges.

Step 6: Scale-Up 

The scale-up phase expands water accounting from pilot basins to regional or national scales, integrating all previously used satellites and the WA+ framework. Comprehensive water accounts support integrated water resource management and SDG 6.4.2 reporting (Bastiaanssen et al., 2015). Scaling requires harmonizing methods, increasing computational capacity, and managing large datasets across multiple basins.

Step 7: Continuous Improvement 

Continuous improvement involves updating methodologies, integrating next-generation satellites (SWOT, NISAR), and applying improved retrievals for evapotranspiration and groundwater (Sheffield et al., 2018; UNESCO, 2021). This adaptive approach reduces uncertainty, improves resilience to climate variability, and ensures that water governance remains future proof. Maintaining technical expertise and securing sustained funding are critical challenges for this phase.

As we conclude, we reflect on what space-enabled water accounting means for the future of water governance and for people like Ramesh who depend on it.

Conclusion

Satellite technologies are fundamentally transforming water accounting and auditing by providing high-resolution, near-real-time monitoring of hydrologic variables across multiple spatial and temporal scales. These capabilities allow for more accurate assessment of water storage, surface and groundwater fluxes, and human withdrawals, reducing uncertainties that traditionally hindered water management. The continuity of long-term missions such as GRACE-FO and SWOT ensures consistent and reliable datasets, which are critical for tracking trends, detecting anomalies, and evaluating the impacts of climate variability and human interventions. Furthermore, advancements in sensor technology, data assimilation, and integration with ground-based observations enhance both the precision and comprehensiveness of water information. By enabling transparent and reproducible analyses, satellite-based monitoring strengthens governance frameworks and supports evidence-based policy decisions.

For communities like Ramesh Patel’s in Bundelkhand, these innovations mean that water scarcity can be monitored more reliably, informing better management of groundwater, surface water, and irrigation resources. By combining space-based observations with local knowledge and independent audits, water managers can make evidence-based decisions that ensure sustainable and equitable water access for farmers, cities, and ecosystems alike.