*Influence of Landsat Revisit Frequency on Time-Integration of Evapotranspiration… DOI: http://dx.doi.org/10.5772/intechopen.80946*

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

#### **Figure 5.**

*About 10 representative ETrF curves for the southern Idaho analysis area during year 2000 created by cubic spline interpolation of ETrF for runs 1, 2, and 4.*

*Advanced Evapotranspiration Methods and Applications*

The first baseline model runs used all 21 ETrF images listed in **Table 1** for Idaho and all 20 images listed in **Table 2** for Nebraska. These runs, representing a condition with four traditional Landsat satellites in orbit or two 'double-wide' Landsats providing four images every 16 days, served as baselines for comparing against sparser image data sets. There were seven times in Idaho and six times in Nebraska when image dates were only 1 day apart, as shown in **Tables 1** and **2**, due to the scheduling of the two Landsat systems and geometry of the WRS path system. In cases where images were 1 day apart, we subtracted 2 days from the first image and added 2 days to the second image in the baseline spline model run 1. This was required to keep the spline function from creating large vertical components caused by a time difference of only 1 day. In cases where images were 1 day apart, the additional information afforded by the second image was deemed to be of much less value than if it had been 4 days apart. Four days apart, larger changes would have occurred in ETrF due to vegetation development and wetting conditions in addition to larger differences in cloudiness. Images 1 day apart typically had similar cloud conditions and ETrF behavior.

Four other integration runs were carried out for the Idaho study area as indicated

Nebraska runs 2 and 3 were made using a combination Landsat 5 and Landsat 7 images from only path 30 or from only path 29. These runs represent scenarios, where two Landsat satellites are in orbit so that the revisit time is each 8 days. Model runs 4 and 5 applied Landsat 5 and Landsat 7, respectively, to path 29, only. Each of these runs represented conditions where only a single Landsat is in orbit, with revisit of 16 days for the majority of a path area. This represents the actual scenario for the USA during the late 1980s and 1990s when only Landsat 5 was collecting data and again in 2012 when only Landsat 7 was operational. Model run 4 was setup to only process imagery from Landsat 5 for path 29 and model run 5 was setup to

**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

in **Table 1**. These runs represented conditions where fewer than four revisits per 16-days were available. Runs 2 and 3 were made using Landsat 5 and Landsat 7 images from only one path, either path 39 or path 40. These runs represent scenarios where two Landsat satellites are in orbit and the focus includes the center two-thirds of a path so that the revisit time is each 8 days. Runs 2 and 3 represent two replicates of the same scenario of 8 day revisit, which is possible in the path overlap area. Run 4 for the Idaho study represents the scenario presented when only one Landsat is in orbit, collecting data every 16 days. This represents the actual scenario for the USA during the late 1980s and 1990s when only Landsat 5 was collecting data and again in 2012 when only Landsat 7 was collecting data. Run 4 was constructed by using imagery for path 40 and Landsat 5. Additional runs 5, 6, and 7, would have represented three additional replicates of a single satellite having 16-day revisit, via combinations of path 40 with Landsat 7 and path 39 with Landsat 5 and path 39 with Landsat 7. However, runs 5, 6, and 7 were not possible to implement because too few images were available during the April–October to apply the ETrF interpolation process without applying what was considered to be too much specu-

lation on the evolution and trends in ETrF over time.

only process imagery from Landsat 7 for path 29.

**2.4 Model runs**

**58**

**3. Results**

**3.1 Splining results**

**Figure 6.**

*About 10 additional representative ETrF curves for the southern Idaho analysis area during year 2000 created by the cubic spline interpolation of ETrF for runs 1, 2, and 4.*

in Idaho. Other curves reflect behavior for alfalfa crops that are harvested three to five times per growing season so that the ETrF curves fluctuate up and down over time. The higher frequency imagery in run 1 was able to capture more of the impacts of harvest and regrowth of alfalfa on the ETrF values. Both runs 2 and 4 missed some of the alfalfa regrowth cycles, for example in the top right graph in **Figure 6**. Run 4 with only 7 image dates generated smoother ETrF curves due to the more sparse data points. The smoother curves tended to average out variation in ETrF caused by variation in water availability or variation in evaporation from soil following irrigation or precipitation wetting events.
