**3. Results**

### **3.1 Splining results**

**Figures 5** and **6** are examples of daily ETrF curves for the April–October period in Idaho created by the spline interpolation process for 20 sample locations

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**Figure 5.**

*spline interpolation of ETrF for runs 1, 2, and 4.*

*Influence of Landsat Revisit Frequency on Time-Integration of Evapotranspiration…*

representing 20 agricultural fields and crops. The ETrF curves represent the anticipated ETrF on any given day, given the ETrF information input to the splining process. Three of the integration runs are plotted in the figures: the full base run (run 1) containing 21 ETrF data points from 21 image dates, run 2 representing a two-Landsat system in the middle of a WRS path having an eight-day revisit schedule, and run 4 representing a one-Landsat system in the middle of a WRS path having a 16-day revisit schedule. Run 2 contained 9 ETrF data points from 9 image dates and run 4 contained only 7 ETrF data points from 7 image dates. The ETrF curves, which represent the ratio of actual ET to the ASCE Penman-Monteith-based reference ET, are characteristic of crops grown in southern Idaho, where ETrF is relatively low during spring prior to vegetation development, when most ET stems from evaporation from wet soil. ETrF increases during late spring and early summer toward 1.0, representing near maximum ET rates from vegetation that fully covers the ground, and then decreases during fall as crops mature and die or are harvested. Some of the ETrF curves in **Figures 5** and **6** exhibit impacts of evaporation from late summer irrigations following harvest of crops. This is a typical cultural practice

*About 10 representative ETrF curves for the southern Idaho analysis area during year 2000 created by cubic* 

*DOI: http://dx.doi.org/10.5772/intechopen.80946*
