Impacts of center pivot irrigation system uniformity on growth of potato crop and residual soil nitrogen

Maintaining the homogeneity of soil nitrogen(N)and plant vigor across agricultural fields is a major concern for farmers and agricultural scheme planners,particularly fields that are irrigated through pressurized systems,such as center pivots.Therefore,this study was carried out on a 30 hm2 potato field located 650 km south of Riyadh,the capital city of the Kingdom of Saudi Arabia,to investigate the impacts of the center pivot irrigation distribution uniformity on the crop development and the spatial distribution of residual soil N.Irrigation performance test was designed to investigate water application rate and distribution uniformities.The overall water application uniformity coefficients(Cu),determined through Christiansen(Cud)and Heerman(CuH)methods,were determined at 81.29%and 80.64%,respectively.However,the overall water distribution uniformity(Du)was determined at 70%.A considerable variability in the distribution uniformity of irrigation water was observed across the experimental field(a Du value of 67%over the medium spans compared to a Du value of 88%over the inner spans).Results of this study showed a linear correlation between the irrigation water distribution uniformity and the soil N(R^(2)=0.88).On the other hand,the vegetation cover distribution,indicated by the Cumulative Normalized Difference Vegetation Index(CNDVI),was not found to be much responsive to the irrigation distribution uniformity(R^(2)=0.11).A time series of successive NDVI maps extracted throughout the potato crop growth stages showed a consistent trend in the distribution of NDVI across the field,with R2 values that ranged between 0.25-0.73.

Sub-pixel analysis to enhance the accuracy of evapotranspiration determined using MODIS images

A study was carried out to estimate the actual evapotranspiration(ET)over a 1074 km2 of the humid area of Perak State(Malaysia),where water and evaporation cycle deeply influences the climate,natural resources and human living aspects.Images from both Terra and Aqua platforms of the Moderate Resolution Imaging Spectroradiometer(MODIS)sensor were used for ET estimation by employing the Surface Energy Balance Algorithm for Land(SEBAL)model.As a part of the accuracy assessment process,in-situ measurements on soil temperature and reference ET(ET0)were recorded at the time of satellite overpass.In order to enhance the accuracy of the generated ET maps,MODIS images were subjected to sub-pixel analysis by assigning weights for different land surface cover(urban,agriculture and multi-surface areas)reflections.The weighting process was achieved by integrating ET from pure pixels with the respective site-specific ET0 of each land cover.The enhanced SEBAL model estimated ET exhibited a good correlation with the in-situ measured Penman-Montieth ET0,with R2 values for the Aqua and the Terra platforms of 0.67 and 0.73,respectively.However,the correlation of the non-enhanced ET maps resulted in R2 values of 0.61 and 0.68 for the Aqua and the Terra platforms,respectively.Hence,the results of this study revealed the feasibility of employing the sub-pixel analysis method for an accurate estimation of ET over large areas.