Abstract: Using Penman's Formula, we computed the natural evaporation Er of Ningxia districts, and studeid its distribution. The ratio Er/r (aridity index) is used as index for agroclimatological division. The formula Er=KiE0 is used to compute E0 for spring, summer and autumn. The proportion coefficient (jt=fc/E0) between E0 and field evaporation fc is used as a moisture index for major growth periods of wheat, paddy rice and millet. Based on these information, irrigation requirements are studied.