Another major anthropogenic activity that could and is greatly affecting the water cycle balance is agriculture and food production- these two sectors if not sustainable maintained can greatly reduce the amount of water stored away especially in groundwater aquifers. Increasing global populations means an increase in the production of food and thus food consumption. According to Lenntech a significant amount of water needed for the agriculture sector and that water is provided by precipitation and groundwater.
It is said that about 70% of water extracted from groundwater or rivers(freshwater) is used in irrigating agricultural fields. Globally about 3600km3 of the fresh water supply is for human consumption, when about half is already utilized due to evaporation of plants and crop intake along with transpiration. For food production we need to consider the amount of water that is needed for food on an individual basis- on average 2000 kilocal of food is needed for an induvial daily which equates to 1000m3 of water for a year. Thus, a global population of 8 billion people will need 8000km3.
Globally irrigation of water and water withdrawals are estimated at 2000 to 2500 km3 yearly. Many agricultural based countries do not practice sustainable or green agricultural practices. Take for example India, which one of the world’s largest food and agriculture producers, which is greatly reliant on groundwater abstraction and irrigation which has been over exploited through the years. The agricultural sector of India is used to sustain its growing population of over I billion persons and also catering a large amount for example milled rice ranging over 4.4 million tonnes. Thus, over use of groundwater has led to the decline in the amount of groundwater available to supply and thus replenish the waters in rivers, lakes, streams or wells. This problem is further exacerbated due to climate change and decline precipitation.
From the 1960’s due to the introduction and utilization of the method proposed through Green Revolution, India became heavily reliant on irrigation and thus increased irrigation practices – commercially and through household use. Persons would use at home irrigation systems along with pumps connected to groundwater from wells situated around the country. According to Nature volume 460, pages 999–1002 (20 August 2009) In India due to the over use of groundwater stores, from the year 1980 studies show that ground levels have decreased and dropped from 8 mbgl to 16 mgbl in the north parts of India, while the rest of the country has seen a decline from 1 mbgl to 8 mgbl. The north western parts of India have lost between the years of 2002-2008 109km3 of groundwater, it is stated that this volume of water lost is two times the amount of India’s largest water reservoir. Studies done have also seen trends showing significant decline in the rainfall from since 1961, which is very much pronounced in the Northern parts of India, which as stated previously is one of the major agricultural areas in India.
An IMD study was done to investigate both the seasonal and annual rainfall for 632 districts along with 34 meteorological sub divisions in India from 1901-2013. It was found that 32 out of the 75 districts which accounts for more than 40% of the districts showed great reduction in the amount of rainfall received. With the over abstraction of water due to agriculture, domestic use or reduction in rainfall, the levels of water in groundwater can be significantly affected and thus the amount of water stored in that reservoir reduced. One way in which groundwater levels are being increased and improved is using the MAR system – Managed aquifer recharge. This is an artificial system which allows for the management of water to essentially maximize and increase the natural storage capacity of groundwater to be used during times of drought or low water flows.
This process involves constructing various infrastructure and modify the land to allow for the intentional increase of groundwater recharge. This system has been used in countries like Bangladesh which over the years has become greatly water stressed due to vulnerability brought by climate change and its effect on the water cycle. The MAR system has proven beneficial in allowing water to be stored for future use, recharging groundwater stores due to over exploitation and the use and reuse of waste water.
MAR systems can utilize several different methods to allow for ground water recharge which include injection wells, ponds, basins or trenches. One method is called the spreading method where the use of infiltration ponds are utilized and overflows from these ponds during excessive rainfall are dispersed in a manner that will recharge groundwater. There is also the use of the in-channel modification methods where sand dams, leaky dams, subsurface harvesting systems (percolating ponds), where river channels are changed in a manner to allow for recharge increase. Thirdly the use of wells is used where infrastructure is built and pump water into the aquifer and will allow recharge.
Rain water harvesting is also utilized, it is captured using an impervious surface stored in either an above ground or below ground tank and then the water is slowly allowed to infiltrate into the ground. Lastly is the induced bank infiltration method where the ground water is removed from one place to create a hydraulic gradient which will cause an increase recharge rate.