BWS-China: WRI’s New Water Stress Map

By Dr. Jiao Wang, Dr. Lijin Zhong, Dr. Ying Long 20 July, 2016

WRI on their new & more detailed China water stress map

The World Resources Institute (WRI)’s Aqueduct Baseline Water Stress (“BWS-Global”) indicator, a central component of Aqueduct’s Water Risk Atlas, provides an overview of the total demand for surface water across sectors and the available annual renewable surface water supply in a given place.

BWS-Global has attracted a large group of users including companies and investors, researchers, NGOs, consultants, international organizations, and governments.

BWS-Global uses water withdrawal data at country level

Mapping the entire globe, BWS-Global uses water withdrawal data at country level, as reported to the Food and Agriculture Organization of the United Nations (FAO), which are then spatially disaggregated across use sectors by catchment area.

While more detailed water withdrawal or demand data (e.g. higher spatial resolution, more temporally frequent) data are available in some countries, they are often not universally available or use different units of analysis or inconsistent methodologies.

But in China, data is available at the prefecture level for a total of 345 administrative subdivisions …

For example, in the US, water withdrawal data is available at county level, while in China, they are available at the prefecture level for a total of 345 administrative subdivisions nationally. Some spatial patterns can be lost when an aggregated number is used at the country level, especially when the regions within a country have distinct water withdrawal characteristics due to their economic and social development differences.

To reduce or avoid the loss of spatial pattern information which is important for estimating water risk accurately, it is important to use more detailed data for a country-specific purpose when they are available. WRI did just this with BWS-China.

BWS-China: WRI’s next level of water mapping in China

In an effort to respond to this need, WRI has developed BWS-China, a mapping tool that provides spatial analyses on baseline water stress in China.

The main distinction is that BWS-Global uses FAO AQUASTAT country-aggregated data, while BWS-China uses more spatially detailed data from govt sources

Developed based on the BWS-Global with some modifications, BWS-China and the BWS-Global have both similarities and distinctions. The primary similarity is that both BWS-China and BWS-Global use the same method for calculating water supply, or “total available blue water,” or, the total annual renewable surface water supply available at that point (for details see here).
The primary distinction is that BWS-Global uses FAO AQUASTAT country-aggregated water withdrawal data and spatially disaggregates it to sub-national level; while BWS-China uses more spatially detailed water withdrawal data from official government sources.

To construct BWS-China, two measures of water withdrawals (water withdrawal and consumptive use) are required:

• Water withdrawal: is the total amount of water abstracted from freshwater sources for human use. Water withdrawals data by sector (domestic, industrial, and agricultural) in China were derived from the Chinese Water Resources Bulletin (WRB) published by each Chinese province. In China, the Water Resources Department in each province is responsible for collecting water resources data within that province and publishing the WRB every year on the department’s website. Water withdrawal data are collected from surveys reported by source and representative sampling, and compiled for each prefecture as a whole. In total, there are 369 prefectures in China, excluding Hong Kong, Macau, and Taiwan. As a contrast, the BWS-Global used sectoral water withdrawal data at the country level.
• Consumptive use: is the portion of water evaporating or being incorporated into a product, and no longer available for downstream use. Consumptive use is derived from total withdrawals based on ratios of consumptive use to withdrawals developed by Shiklomanov and Rodda.¹

Calculating water withdrawal

Following the same analytical methodology in BWS-Global, water withdrawals for the year 2010 were spatially disaggregated by sector based on spatial datasets. All spatial datasets had a resolution of 1 square kilometre.

• Agricultural water withdrawals were disaggregated using irrigated areas data
• Industrial water withdrawals using factory gross output
• Domestic water withdrawals using population density data (click on image to enlarge)

The BWS Map also calculates Total withdrawal, which is the total amount of water removed from fresh water sources for human use. The total withdrawal is the sum of agricultural, industrial and domestic water withdrawals (so the three maps above) at total catchment level. . Figures 1, 2, 3 displays total catchment level water withdrawal intensity by agricultural, industrial, domestic sector, respectively. The map below displays total water withdrawal intensity (all sectors) at the catchment level.

Total withdrawal is the total amount of water removed from fresh water sources for human use

Calculating water consumptive use

Consumptive use is the proportion of all water withdrawn that is consumed through evaporation, incorporation into a product, or pollution, and therefore no longer available for reuse. Consumptive use by sector is estimated from total withdrawal using consumptive use ratios by Shiklomanov and Rodda.¹  The map below displays consumptive use intensity at the catchment level.

Consumptive use is the proportion of all water withdrawn that is consumed and therefore no longer available for reuse

Calculating China’s Freshwater Baseline Water Stress

Baseline Water Stress (BWS) is the annual water withdrawals (domestic, industrial, and agricultural) divided by the mean of available blue water (surface). So, before can calculate China’s new BWS we need to calculate its available blue water.

Available blue water (Ba) is the total amount of water available to a catchment before any uses are satisfied. It is calculated as all runoff water from upstream catchments minus upstream consumptive use plus runoff in the catchment. Ba is calculated as  where R is runoff, Qout is the volume of water exiting a catchment to its downstream neighbour: Qout(i) = max(0, Ba(i)-Uc(i)), Uc(i) are the consumptive uses. Negative values of Qout are set to zero.²

China’s available blue water intensity at the catchment level is displayed in the map below.

Available blue water is the total amount of water available to a catchment before any uses are satisfied

Now to move onto BWS. It is a chronic measurement of the level of competition and depletion of available water, and is a good proxy for measuring water risks more broadly.³

BWS is a chronic measurement of the level of competition & depletion of available water
A higher value = more competition for water & depletion of water resources

A higher value indicates more competition for water among users and depletion of water resources. These ratio values were then grouped into Baseline Water Stress classifications based on the methodology used in BWS-Global: low (<10%), low to medium (10 – 20%), medium to high (20 – 40%), high (40 – 80%), and extremely high (>80%).

In BWS-China, BWS was calculated for the year 2010 as the total water withdrawals from year 2010 divided by mean available blue water. A long time series of runoff data from 1950 to 2010 was used to reduce the effect of multi-year climate cycles and the complexities of short term water storage (e.g., dams, floodplains).⁴

Being consistent with the BWS-Global classification, areas with available blue water and water withdrawal less than 0.03 and 0.012 m/m² respectively were classified as “arid and low water use.”² The map below displays BWS at the catchment level. The white patches located within grey areas (i.e. arid and low water use) are lakes and ponds which are not delineated as a catchment in the GDBD dataset.

Comparing results: BWS-Global vs. BWS-China

As noted in the box above, a major difference between the BWS-Global and BWS-China is the industrial water withdrawals disaggregation methodology: BWS-Global used night-time lights, while BWS-China used industrial factory locations and their gross output.

BWS-Global used night-time lights … while BWS-China used industrial factory locations & gross output

BWS-China and BWS-Global are compared side by side below (click on image to enlarge).

Compared with nighttime lights, industry factory locations provide more detailed and accurate information on the likely location of industrial water withdrawals. For example, although nighttime lights are used as a proxy for industrial water use, the dataset also captures streets and roads with lights where industrial water withdrawals do not occur.

Overall results show BWS-Global and BWS-China share similar spatial patterns, and, as expected, both show that the relatively arid northern region of China experiences more stress than China’s wetter southern regions.

However, a closer look at the catchment level reveals differences. For example, BWS-China shows less stress than BWS-Global in the downstream areas of the Yellow River.  This reflects BWS-Global’s overestimation of water withdrawals, particularly the industrial sector.

BWS-China shows less stress than BWS-Global in downstream areas of the Yellow River
And higher stress in the river mouth areas of the Yangtze River

BWS-China shows higher stress than BWS-Global in the river mouth areas of the Yangtze River. In this case, BWS-Global significantly underestimated domestic withdrawal.  These differences are attributed to the more detailed water withdrawal data and higher resolution used to develop BWS-China.

We will compare the BWS-Global map and BWS-China map in more detail in a future publication.

For the first time more detailed, country-specific data having been substituted into the BWS-Global indicator. BWS-China provides a useful model for other stakeholders wishing to develop BWS assessments using locally relevant datasets in their countries. Users are encouraged to: use BWS-Global for a global understanding of water stress and comparison across countries and larger regions; and use BWS-China for more detailed, geographically specific information on water stress in China.

BWS-China is useful for investors, companies, govt agencies & more

Investors, companies, government agencies and others whose interest is mainly focused within China can use BWS-China to evaluate investment opportunities and/or dig deeper into understanding potential water risks and begin to address these challenges.

For the full paper, click here.