The Ogallala aquifer described in the Closer Look box in Section 18.3, provides 82% of the drinking water for the people who live in the region, although more than 75% of the water that is pumped from it is for irrigation. Irrigation withdrawals are approximately 18 billion gallons per day. (a) Assuming that 2% of the rainfall that falls on an area of 600,000 km2 recharges the aquifer, what average annual rainfall would be required to replace the water removed for irrigation?

Verified step by step guidance

1. Convert the daily irrigation withdrawal from gallons to cubic meters using the conversion factor: 1 gallon = 0.00378541 cubic meters.
2. Calculate the annual irrigation withdrawal by multiplying the daily withdrawal in cubic meters by the number of days in a year (365 days).
3. Determine the volume of water that needs to be recharged annually by dividing the annual irrigation withdrawal by the recharge percentage (2%).
4. Convert the area of 600,000 km^2 to square meters using the conversion factor: 1 km^2 = 1,000,000 m^2.
5. Calculate the average annual rainfall required by dividing the volume of water that needs to be recharged by the area in square meters. This will give the rainfall in meters, which can be converted to other units if needed.

Key Concepts

Aquifer Recharge

Aquifer recharge refers to the process by which water from precipitation or surface water infiltrates into the ground and replenishes underground water reserves. In the context of the Ogallala aquifer, understanding how much rainfall contributes to this recharge is crucial for assessing the sustainability of water resources, especially when a significant portion is used for irrigation.

Water Balance

Water balance is a hydrological concept that accounts for the inputs and outputs of water in a given system. It involves calculating the total amount of water entering (through precipitation and recharge) and leaving (through evaporation, transpiration, and withdrawals) the system. For the Ogallala aquifer, maintaining a positive water balance is essential to ensure that the aquifer can sustain its water supply for both drinking and irrigation.

Irrigation Water Use

Irrigation water use refers to the amount of water extracted from sources like aquifers for agricultural purposes. In the case of the Ogallala aquifer, understanding the scale of irrigation withdrawals (18 billion gallons per day) is vital for calculating the necessary rainfall to replenish the aquifer. This concept highlights the significant impact of agricultural practices on water resources and the need for effective management strategies.