In Texas, on average, the hottest three weeks of the year are the last week of July and the first two weeks of August. When we get days with peak temperatures over 100 degrees, they are most often concentrated in this three week period. Late summer weather is often dominated by high pressure domes that sit over New Mexico and west Texas. The domes tend to be associated with weather patterns that bring in hot dry winds from New Mexico and the central plains states north of Texas. To survive, some native small plants often go dormant in the late summer while larger plants such as trees use more water to cool themselves.
The combination of heat, low humidity, and maximum water use by trees causes water loss from soils to peak in late summer. An additional factor is that the typically low rainfalls in June and July have already allowed a great deal of water to be pulled out of our soils by evaporation. Before the peak drying season starts, our soils are often very dehydrated.
As we enter the late summer, it is very common for the soils to have contracted and cracked as the result of water loss. As soils crack, hot dry air can penetrate into the ground further accelerating water loss. On hot dry windy days, standing water can evaporate at the rate of one inch per square foot per day. Your yard is not a pond, so the rate of water loss is lower. If you lose 1/2 of an inch of water per day per square foot, that works out to about 1/3 of a gallon per square foot per day. For a 2,000 square foot yard, you are losing about 625 gallons of water per day or almost 20,000 gallons a month. Water loss to evaporation is greatly increased by spraying. Depending on conditions, it is entirely possible to lose twenty percent of the water that you are spraying to evaporation.
To combat water loss while minimizing your expense, the best route is to use soaker hoses, drip hoses, or hoses with emitters (small openings that allow a small amount of water to escape). Use timers and water at night. Temperatures are lower and humidity is higher at night. Such conditions will allow more time for the water that you apply to soak into the ground. Avoiding spraying water through the air will reduce the amount of water you use. At the same time, you want to concentrate the water where you need it, around your foundation. Sprinkler heads that are designed to spray water over large areas are not an effective way to water the soils that lie below your foundation.
Water moves from areas of greater concentration to areas of lower concentration. As the soils in your yard dry out the water in the moist soils under your home begins to be pulled out. By placing soaker hoses around the edge of your foundation you can create a curtain of damp soil that will slow or stop the loss of water from the soils under your foundation. Evaporative loss can be further reduced by burying your hoses a few inches below the soil.
It is important to know that, at their edges, most foundations extend from one to three feet into the ground. (This is a generalization. Depending on design parameters, some residential foundations have perimeter beams that extend as far as seven feet into the ground.) For the drying of soils to affect your foundation, the drying must affect the soils down to a depth below the edge of your foundation. For a watering program, this means that you must get the water down to the lower edge of your foundation, typically one to three feet deep.
Of course, water will not simply flow down into the ground. Water must slowly work its way down. Given the daily loss to evaporation, this means that you should consider watering your foundation daily. You should apply enough water to thoroughly wet the surface soils ( the top 6 to 12 inches) so that the free water can be pulled down into the deeper soils. Once water that you are applying works its way down to the lower edge of your foundation, water migration from under your slab will slow or stop.
During the summer, as the soils in your yard dry, they will shrink and crack. It is very common for the soil in your yard to pull away from the side of your foundation. Such separations can be over an inch wide and, at times, several feet deep. If separations are developing between the soils in your yard and your foundation, do not be alarmed, just continue watering.
You should continue regular watering until temperatures drop and rains come.