Crop yield is affected by crop condition. With a favorable crop condition,better crop yield can generally be expected;otherwise,crop yield will be lower. Thus,crop condition monitoring is very important. Normalized Difference Vegetation Index（ NDVI） is widely used to monitor crop condition by global agricultural monitoring systems. In irrigated and rain-fed mixed agricultural zones,the average NDVI will exaggerate the water stress on irrigated cropland and ignore the water stress on rain-fed cropland,possibly misleading a policy maker. Therefore,crop condition monitoring in rain-fed cropland should be separated from that in irrigated cropland,and the crop conditions in rain-fed and irrigated croplands in the same crop zones should be compared. As a typically irrigated and rain-fed mixed crop zone,Nebraska in the United States was selected for the quantitative analysis of the difference between crop conditions in irrigated and rain-fed croplands.First,this work selected the drought（ 2012）,normal（ 2005）,and rainy（ 2008） precipitation years by calculating the accumulative frequency of rainfall through the Tropical Rainfall Measuring Mission（ TRMM） precipitation time series from 2001 to2013. Second,the cropland of Nebraska was divided into rain-fed（ IFC < 30%）,mixed（ 30% ≤ IFC < 60%）,and irrigated（ IFC≥60%） lands according to the cropland irrigated fraction dataset. Third,the difference in maximum NDVIs and the similarity of NDVI time series profiles of the different groups（ irrigated,mixed,and rain-fed zones） were analyzed in 2005,2008,and2012. Lastly,this work analyzed the NDVI change pattern with the increase of irrigation fraction in 2005,2008,and 2012.The results are as below:（ 1） The crop condition tends to be better with an increase in irrigation fraction at any year; an increase in NDVI accelerates faster when irrigation fraction is less than 60%,but becomes slower when irrigation fraction is larger than 60%.（ 2） The similarity of NDVI time series becomes strong with an increase in irrigation fraction at any year,indicating that variation in crop conditions eases with an increase in irrigation fraction.（ 3） In the drought year（ 2012）,the development trend of the NDVI profile is similar with that of the rainfall profile,whereas maximum NDVI was lagging behind maximum rainfall because of the influence of irrigation water; in the rainy year（ 2008）,the development of NDVI was consistent with that of rainfall owing to the alleviation of water stress.（ 4） The contribution of irrigation to crop condition in drought year（ 2012） is larger than those in normal and rainfall-abundant years.In consideration of the crop condition differences in rain-fed,mixed,and irrigated croplands in different years,crop condition monitoring in an irrigated crop zone should be carried out separately from that in a rain-fed crop zone.

irrigated agricultural zone;mixed agricultural zone;rain fed agricultural zone;crop condition;NDVI