目前缺水已成为制约作物产量进一步提高的主要因素，如何提高水资源利用效率(WUE)已成为我国农业亟待解决的问题。为研究玉米产量和水分利用效率对气象因素及群体变化的响应，我们在全国25个试验点安排了不同玉米群体的多点试验，不同的玉米群体由不同株型玉米品种(紧凑型和半紧凑型)和不同种植密度(30000株/ha、60000株/ha和90000株/ha)构成。研究发现：随着降雨量的增加，玉米产量呈现先增加后降低的趋势，水分利用效率随着降雨量的增加显著降低，60000株/ha种植密度的产量和WUE>90000株/ha>30000株/ha。通过分析WUE与主要气象因子的关系发现：水分利用效率与在各生育阶段(播种-吐丝、吐丝-成熟/收获、播种-成熟/收获和全年)的降水量(和R)呈显著负相关，与温度(T_m、T_M–m和GDD)、太阳辐射(Ra)呈正相关。为了研究不同玉米群体的水分蒸散特征，我们进一步安排了不同玉米群体的定点试验，研究发现：随着玉米群体的增大，玉米需水量显著增加，而土壤蒸发量显著减少。不同株型品种间及不同种植密度间玉米需水量和土壤蒸发量也存在显著差异。分析品种和种植密度对WUE的影响发现：选择抗旱杂交品种和适宜种植密度WUE分别提高21.70%和14.92%，表明选用抗旱杂交种比改变种植密度对WUE的提高作用更显著。综合考虑气象影响，采用ZD958等抗旱品种和60000株/ha的种植密度是提高我国北部玉米产量和WUE的有效途径。

Crop consumptive water use is recognized as a key element to understand regional water management performance. This study documents an attempt to apply a regional evapotranspiration model (SEBAL) and crop information for assessment of regional crop (summer maize and winter wheat) actual evapotranspiration (ETa) in Huang-Huai-Hai (3H) Plain, China. The average seasonal ETa of summer maize and winter wheat were 354.8 and 521.5 mm respectively in 3H Plain. A high-ETa belt of summer maize occurs in piedmont plain, while a low ETa area was found in the hill-irrigable land and dry land area. For winter wheat, a high-ETa area was located in the middle part of 3H Plain, including low plain-hydropenia irrigable land and dry land, hill-irrigable land and dry land, and basin-irrigable land and dry land. Spatial analysis demonstrated a linear relationship between crop ETa, normalized difference vegetation index (NDVI), and the land surface temperature (LST). A stronger relationship between ETa and NDVI was found in the metaphase and last phase than other crop growing phase, as indicated by higher correlation coefficient values. Additionally, higher correlation coefficients were detected between ETa and LST than that between ETa and NDVI, and this significant relationship ran through the entire crop growing season. ETa in the summer maize growing season showed a significant relationship with longitude, while ETa in the winter wheat growing season showed a significant relationship with latitude. The results of this study will serve as baseline information for water resources management of 3H Plain.

The North China Plain (NCP), one of the most important agricultural regions in China, is facing a major water-resource crisis evoked by excessive exploitation of groundwater. To reduce water use while maintaining high crop production level, improving variety water use efficiency (WUE) is an urgent need, especially because other water-saving measures such as water delivery, irrigation, and agricultural practices have already achieved most possible progresses. Evaluation of variety WUE can be performed accurately at the individual plant level (WUEp). Reviewing the studies on physiological factors affecting WUEp performed up to date, stomatal conductance was considered to be an important trait associating closely with WUEp. The trait showed a large degree of varietal variability under well-watered conditions. Crop varieties differ highly in sensitivity of stomata to soil and air drying, with some varieties strongly reducing their stomatal conductance in contrast with those lightly regulating their stomata. As a result, difference among varieties in WUEp was enlarged under water deficit conditions in contrast with those under well-watered conditions. The relationship between stomatal conductance and yield depends on water availability of whole growing period in local areas. Usually, large stomatal conductance results in a high yield under good irrigation system, whereas a low stomatal conductance can lead to yield benefit under limited stored soil moisture conditions. In the NCP, winter wheat is the largest consumer of irrigation water, improvement strategies for high WUE aiming at wheat crops are in urgent need. We suggest, for the well-irrigated areas with excessive exploitation of groundwater, the wheat breeding program need to combine medium stomatal conductance (0.35 mmol H2O m-2 s-1 or so), high carboxylation efficiency, and high harvest index. Areas with partial/full access to irrigation, or infrequent drought, should target wheat varieties with high stomatal conductance under no water stress and low sensitivity of stomata to soil water deficit. Drought-prone rain-fed areas characterized by frequent and long terminal drought should target wheat varieties with low stomatal conductance under no water stress and high stomata sensitivity to soil drying to make water available during grain filling.

Climate change will have important implications in water shore regions, such as Huang-Huai-Hai (3H) plain, where expected warmer and drier conditions might augment crop water demand. Sensitivity analysis is important in understanding the relative importance of climatic variables to the variation in reference evapotranspiration (ET0). In this study, the 51-yr ET0 during winter wheat and summer maize growing season were calculated from a data set of daily climate variables in 40 meteorological stations. Sensitivity maps for key climate variables were estimated according to Kriging method and the spatial pattern of sensitivity coefficients for these key variables was plotted. In addition, the slopes of the linear regression lines for sensitivity coefficients were obtained. Results showed that ET0 during winter wheat growing season accounted for the largest proportion of annual ET0, due to its long phenological days, while ET0 was detected to decrease significantly with the magnitude of 0.5 mm yr-1 in summer maize growing season. Solar radiation is considered to be the most sensitive and primarily controlling variable for negative trend in ET0 for summer maize season, and higher sensitive coefficient value of ET0 to solar radiation and temperature were detected in east part and southwest part of 3H plain respectively. Relative humidity was demonstrated as the most sensitive factor for ET0 in winter wheat growing season and declining relativity humidity also primarily controlled a negative trend in ET0, furthermore the sensitivity coefficient to relative humidity increased from west to southeast. The eight sensitivity centrals were all found located in Shandong Province. These ET0 along with its sensitivity maps under winter wheat-summer maize rotation system can be applied to predict the agricultural water demand and will assist water resources planning and management for this region.