Reference: Hao, Lu, Xiaolin Huang, Mengsheng Qin, Yongqiang Liu, Wenhong Li, Ge Sun. Ecohydrological Processes Explain Urban Dry Island Effects in a Wet Region, Southern China. Water Resources Research, 54:6757-6771. https://doi.org/10.1029/2018WR023002.
Cover image of the Shanghai, China skyline at night provided under a creative commons license by Romain Pontida and downloaded from flikr.
Human settlements are made up of more than just a parking lot. The largest are massive groups of cities with millions of residents and the attendant roads, parking lots, and buildings that support thriving economies. But before we try to understand what happens to the temperature and water cycles when increasingly large areas of the surface of the Earth are paved and developed for human use, let us first consider the parking lot.
The Parking Lot
We know that when a parking lot—plus, as the song goes, a pink hotel, a boutique and swinging hot spot—replaces trees, the water movement also changes. In a forest, water might arrive as rain, fall to the ground, and soak into the soil, recharging groundwater. Groundwater, in turn, is pulled up through the roots and branches of trees and expelled into the atmosphere as a byproduct of the tree’s metabolism in a process known as evapotranspiration.
Pavement, on the other hand, creates a barrier, or lid, on the ground that water cannot penetrate. Instead of going into or moving out of the ground, water moves away, often by design, and often quickly, sped up by smooth, hard, urban surfaces. Pavement also stores and releases heat differently than the forest. With less water to move, pavements and buildings absorb more energy, get hotter, and then release that energy more slowly. This contributes to a phenomenon called “urban heat island,” where the temperature of the air in cities is hotter than the air in the surrounding natural environment.
Figure 1. Simulated temperature profile of an urban-rural cross section showing cooler rural and warmer downtown temperatures. The urban heat island is measured by taking the difference between the minimum (rural) and maximum (urban) temperatures. Urban heat islands are usually strongest at night under clear, calm conditions. Image provided by wikimedia commons.
From Hot to Dry
Temperature and water are closely linked. Changing water from a liquid to a gas takes a lot of energy, a material property known as specific heat. The specific heat of water is much higher than most other common substances. As a result, water plays an important role in regulating temperature.
While urban heat islands are easy to measure and are found in cities of various sizes around the world, it’s harder to measure what is happening to the water, particularly the water vapor. The question is: does the paved, developed surface of the urban environment mean that less water is entering the atmosphere, creating what would be called an “urban dry island” effect?
Because air moves and new air brings in new water vapor, it is really hard to measure this relationship for a parking lot, or even a small city. As cities get larger, however, the ability to detect this phenomenon becomes more likely.
To test the presence of an urban dry island, Hau and colleges used the mega urban region around Shanghai, China as their study site. The two provinces, Jiangsu and Zhejiang, are home to some 111 million urban residents in three large cities, plus another 48 million rural residents. The land area covers just 1% of China’s total land area, or 210,700 square kilometers. Between the years of 2000 and 2010, the urban area in the study region increased by 75%, resulting in a loss of 15% of the surrounding rice paddy area, 9% loss of dry crop land, and 14% loss of grassland.
Using data from satellite imagery and data from 33 standard weather stations across the region, the researchers tested several pathways by which the increase in urban area could affect the land-air exchange of water and heat.
The data indicated an increasingly dry urban core, however the data further suggested that temperature was not the driving cause of the dryness. Instead, the data revealed that the conversion of agricultural land to urban land was largely responsible. Specifically, the loss of rice paddy land meant the surface water from flooded fields was no longer present nor the vegetation to pull water from the ground and release it through evapotranspiration to the atmosphere.
The large scale of the urbanization was further noted by researchers as an important aspect of the study. As the scale of land change increases, the impacts on the atmosphere and hydrologic processes also increase. A drier atmosphere can make it harder, for instance, to irrigate the remaining crops.
But just as with the trees in the mythical parking lot, it’s hard to know what you’ve got until it’s gone.