Part of the process includes knowing how much water a given system will apply over a given period of time and how efficiently that system distributes moisture. Studies show LEPA center pivot systems, for instance, may be as high as 95 percent efficient. Drip systems could be a little more efficient, standard center pivots and furrow irrigation will be somewhat less.

Another, and a less readily available figure is the crop coefficient: how much water a specific crop uses in a day during various growth stages.

"We've made improvements in irrigation efficiency over the past few years," says Giovanni Piccinni, a Texas A&M University researcher at Uvalde.

Piccinni, speaking at the recent Texas Produce Convention in South Padre Island, says farmers have used LEPA, furrow dikes, bubblers and drip systems to make better use of available water supplies.

But knowing crop needs promises to improve moisture management even more.

Piccinni uses potential evapo-transpiration rates as a base. "PET data is available on a regional basis throughout the state to guide irrigation decisions," he says. "PET is simply the amount of moisture lost from the soil."

Those rates are available on regional Web sites.

He says establishing crop coefficients to indicate how much moisture a grower needs to replace requires a bit more study. Various crops have different moisture demands. Vegetables, for instance, will require more water than grain sorghum, for instance.

"We don not have adequate crop coefficients for vegetable crops,' Piccinni says. "Most of our work has been done on row crops."

Growers can use tables and equations to figure moisture needs for specific crops, but Piccinni is looking for a more accurate means of evaluating moisture demand.

He's built a weighing lysimeter, basically a 30,000-pound soil-filled box, placed in the middle of a field. He places scales at the bottom of the box to weigh any changes in soil weight. Removing or adding moisture alters the weight.

"The scales will record a weight change within one-fourth of a pound," he says.

Soil inside the lysimeter is the same as the soil in the field. Fertility, pest control and water also will be the same.

"As crop conditions change the scales weigh and record the differences. We collect data 24 hours a day, seven days a week."

Piccinni has a year's worth of research on onions and spinach and is about to put a second crop of each on the lysimeter.

"We've found that we can make a spinach crop with 9 inches of water," he says. "On the last test we applied 6.93 inches. We tried to keep the crop moisture at a constant level. We did not need a lot of irrigation."

He also discovered how much water spinach needs during critical growth stages. At emergence, spinach needs 0.35 inch per day. At the two-to-three-leaf stage, demand increases to 0.45 inch. Heaviest demand comes later and jumps to 1 inch per day during the 16-to 18-leaf stage and from the 19-leaf stage to harvest.

Season moisture demand would be:

Growth Stage crop coefficient
Emergence 0.35
2-3 leaves 0.45
4-6 leaves 0.66
7-9 leaves 0.75
10-12 leaves 0.90
16-18 leaves 1.00
19 - harvest 1.00

Piccinni says yield response to ET rates allows growers to use available water at the most opportune times and may allow them to conserve water.

"We tried to keep onions out of stress," he says. Irrigation applications usually ranged from 1 inch to 1.5 inch.

Growth stage and crop coefficient rates for onions are:

Growth Stage crop coefficient
Emergence 0.50
2 leaves 0.60
3-4 leaves 0.70
5-6 leaves (begin bulbs 0.80
7-9 leaves (bulb develop) 0.85
Bulb developed 0.90
Dry leaf 0.70

Piccinni says research shows that growers do not have to replace all the water lost to ET, another indication that using crop coefficients and PET may save water.

"We have two years of data on corn and one each on onions and spinach and are starting on our second year with those two crops. We may look at cotton next."

e-mail: rsmith@primediabusiness.com