It is also extremely helpful to know when in the year it is most likely to rain. On average, the “typical” year's rainfall in New Mexico will be delivered as follows:
• One-eighth will be harvested in Season 1 (September, October and November).
• One-quarter will fall in Season 2 (typically December, January and February).
• One-eighth will be harvested in Season 3 (March, April and May).
• One-half of the year’s precipitation will come in Season 4 (June, July and August, in most parts of the state).
Based upon recent data, precipitation patterns may be changing and New Mexico’s summer “monsoon” season may not be as predictable or consistent as in the past. Nevertheless, New Mexico continues to receive a large percentage of its annual precipitation during the warmer months.
If 12,000 gallons of water income is anticipated to be harvested per year, the first season (fall), of the water budget year will typically produce 1,500 gallons of new water. During the second season (winter), 3,000 gallons will be collected and the third season (spring) will provide 1,500 gallons. The total new-water income expected during these three seasons is 6,000 gallons.
Let’s assume that your cistern holds 6,000 gallons, and that you start the water budget year (September 1) with a full cistern. In this example, the total anticipated water budget for the first three seasons of the year is 6,000 gallons of new income plus 6,000 gallons of previously stored water, or 12,000 gallons of total water income from September through May. However, since the goal is to start the fourth season of the year with a full cistern (because it is the hottest part of the year), of the 12,000 gallons only 6,000 gallons of it is useable income.
In the example above, the estimated fourthquarter water budget would be 12,000 gallons (6,000 gallons in storage plus the anticipated 6,000 gallons in new water to be harvested during the fourth quarter). Because some years are drier than “normal,” it is wise to estimate usable fourthquarter income at approximately 80% of maximum. Consequently, you should install plant material that can survive on approximately 9,600 gallons of water during June, July and August.