How urban green spaces affect aquifer infiltration: a case study

Urban green spaces are not only a more pleasant living environment and cleaner air. Their development is of great importance to shallow aquifers, increasing the supply of water within drought-prone cities. A scientific study conducted in the Mexican capital quantifies the benefits associated with infiltration.

Aquifers in cities

It has been known for years that concreting and asphalting large areas of cities reduces the ability of aquifers to absorb rainwater and at the same time increases the risk of local flooding. Urban green spaces are supposed to mitigate this risk by providing a wide range of ecosystem services. Researchers from the National Autonomous University of Mexico (UNAM) set out to study how the nature of green spaces determines the level of rainwater infiltration.

The properties of soils used in urban green spaces often result from the aesthetic or recreational function of these areas. Adverse changes in the structure of the soil, occurring as a result of human activities, include such aspects as a reduction in overall porosity, a reduction in macropores and an increase in mesopores, resulting in a decrease in hydraulic conductivity. By the latter we mean the soil’s ability to carry water under saturated or nearly saturated conditions, measured in meters per day. The type of vegetation and the level of evapotranspiration, i.e. the process of evaporation from the soil and green tissues, are also not insignificant for the infiltration potential of the soil.

In areas where a multi-layered groundwater system is observed, soil quality particularly affects shallow aquifers. The importance of the latter, which are generally not treated as a source of water, is often ignored. Meanwhile, according to the authors of a study published April 24 in the journal PLOS Water, it is this shallowest aquifer that is the easiest and cheapest to control and can act as a reservoir for retaining the freshest rainfall. Thus, it increases the availability of water in the city and reduces the risk of flooding.

Water shortage in Mexico’s capital

How urban green spaces affect urban aquifers has been studied in the Mexican capital, a metropolis with a population of more than 22 million and often facing water shortages. By the first quarter of 2024, the situation had become so severe that in some neighborhoods the waterworks operated for only 1-2 hours a day, supplying households with just a few buckets of water each.

Mexico City is an example of a city where unplanned urban development has led to severe hydrological disruption. 71 percent of the total water supply comes from deep aquifers, located 70-500 meters below the surface. The other major source is a shallow aquifer in the south of the city, comprising a basaltic subsoil formed by the eruption of the Xitle volcano in the 3rd century AD. It is within it that a study was conducted to further clarify the relationship between the various elements of the city’s hydrological system and to identify ways to manage water resources.

Infiltration level test

Of the original ^{80-square-kilometer} post-volcanic basalt subsoil, only 22 square ^kilometers have been protected – the rest has been swallowed up in the process of urbanization. Within the remaining ecosystem, the 2.^{37-square-kilometer} Reserva Ecological del Pedregal de San Angel (REPSA) reserve, formally owned by UNAM, was created in 1983. It was in this area, which is one of the most important urban green spaces, that the study in question was carried out. Average annual precipitation here is estimated at 752 mm, while the daily maximum from 2014-2020 was 690 mm.

Measurements of hydraulic conductivity carried out at 19 points on the REPSA site and the nearby university park showed huge variability – from 0.12 to 23.04 m/day. This means that the local rate of water transfer in the soil ranges from 12 cm to 23 m in one day. At 42 percent of the measurement points, infiltration was between 1 and 5 m per day, at 32 percent more than 10 m, and only at 16 percent less than 1 m per day. In addition, the annual evapotranspiration of the study area was estimated at 0.7 percent of the total annual rainwater volume.

The study found that at as many as 89 percent of the measurement points within the reserve, the potential for rainwater infiltration is higher than the historical daily maximum in recent years. At the remaining 11 percent, it was found that un-soaked rainwater drains away due to gravity toward areas with crushed lava rocks. As a result, all rainwater soaks into the shallow aquifer of the REPSA reserve and there is no surface runoff outside the reserve.

Urban green spaces vs. shallow aquifers

How do these results explain the importance of urban green spaces for a city like Mexico? The researchers calculated that the water that soaked into the REPSA boundaries could meet the annual demand of 39,400 people. An additional benefit is the potential for total absorption of rainwater, thereby reducing the risk of local flooding.

According to the study’s authors, the example of the Mexican capital illustrates well the importance of shallow aquifers, which urban green spaces are responsible for saturating. In a city facing chronic water scarcity on the one hand, and threatened by flash floods on the other, the development of green spaces therefore appears to be a priority. However, spatial planning should include a comprehensive analysis of soil properties, subsoil and evapotranspiration – these are the factors that determine the effectiveness of rainwater infiltration into aquifers.

By expanding urban infiltration areas, groundwater recharge can be significantly improved. Urban green spaces should therefore be valued not only for their potential to reduce high summer temperatures, support local biodiversity and provide psychological benefits to residents, but also for their key role in maintaining hydrological balance and preserving ecosystem services in urban areas.