A SPAC model was used to determine theoretically the amount of water applied by drip irrigation under the condition that the integral value of the daily minimum crown potential during a period of N days for drip irrigation equals that for sprinkler irrigation. The N value is the least common multiple of sprinkler and drip irrigation intervals (D_s, D_d respectively) and rainfall intervals (M days). The amount of water applied by drip irrigation when the drip lines are not set above the main root zone are calculated. The results are as follows: -When rainfall is frequent, the amount of water applied by drip irrigation is more than that by sprinkler irrigation, -When rainfall is scarce, small amount of water applied is needed for drip irrigation. When the drip lines are set above the main root zone, the amount of water applied by drip irrigation could be reduced. Horizontal distribution of soil water is uniform for the sprinkler irrigation. For the drip irrigation, soil moisture below the drip line is larger than that in the root zone. The difference is large when the hydraulic conductivity is small and when the distance between the drip lines and the main root zone is increased.
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