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

(3) imagery from only one Landsat 5 from one path only. The first integration run approximated a condition, where four images are collected each 16 days. This is a condition that would occur with four Landsat satellites in orbit with the current path width or with two Landsat satellites in orbit, each having a 'double-wide' path of approximately 300 km. The second and third integration runs approximated the condition where two currently formulated Landsats are in orbit at any one time, for the center of a WRS path. The last condition represents the condition, where only one Landsat satellite is in orbit.

In the Idaho study area, ET was integrated over the April 1–October 31 period to form monthly ET for April through October. The absence of clear images for the study area during late March and early April and during late October and early November for some of the time integration runs required the use of 'synthetic' images to represent ET conditions during these periods. The synthetic images were required to anchor the spline function prior to April and following October. The synthetic images were created for the Idaho study area by applying a daily soil water balance model for a bare soil condition [11] representing surface conditions during Idaho winters and late falls, where nearly all vegetation is dormant due to freezing. The daily soil water balance model used the FAO-56 evaporation model [11] and was applied to 18 weather stations in the region and an evaporation surface was created using inverse distance interpolation. The average ETrF during the late March to early April and from late October to early November periods was determined by averaging the simulated evaporation rates over those periods. Those synthetic images were then used as beginning and ending points for the spline interpolation process.

No synthetic images were required for the full two-Landsat/two-path integration for the Idaho study area, as sufficient image dates during early April and late October were available. In the double satellite/single path integration, however, synthetic ETrF images were required at the end of the growing season to provide endpoints for the cubic spline, and were placed on dates November 01, 2000 (ETrF averaged 0.25) and November 10, 2000 (ETrF averaged 0.1). For the run using only Landsat 5 data and for path 40 only, synthetic images were required at both the beginning and end of the growing season. In this run, four synthetic images were placed on dates March 20, 2000 (ETrF averaged 0.1); March 31, 2000 (ETrF averaged 0.1); November 01, 2000 (ETrF averaged 0.25); and November 10, 2000 (ETrF averaged 0.1) to provide endpoints for the cubic spline.

For the Nebraska study area, ET was integrated over the May–September period, representing the shorter growing season for the predominately corn and soybean crop rotation there, as opposed to the April–October growing period for Idaho crops. As with Idaho, ETrF for bare soil was also estimated for the Nebraska study area using the FAO-56 style evaporation model for the purpose of creating synthetic images for April 1, April 15, October 15, and November 15. These dates and synthetic images were used in each of the time-integration analyses to approximate ET conditions during those general periods so that the spline function could be applied with spans covering May–September.

In both study areas, about 1500 data points were sampled. The points were selected from the interiors of irrigated fields, with one point per field. Pixels were located far from field edges to avoid contamination of thermal pixels from thermal information from outside the field. Nearly all of the Idaho sample locations were in agricultural fields, with about 15 sample points taken from desert rangeland. Irrigated agriculture was emphasized in this study due to its importance in water resources management. In the Nebraska data set, about 100 sample pixels were selected from rangeland and riparian areas each. The rest were from irrigated agriculture.

*Advanced Evapotranspiration Methods and Applications*

**Time integration using both paths and landsats**

L7 \* \*

L5 \* \*

L7 \* \*

L7 \* \*

L7 \* \*

L5 \* \*

L7 \* \*

L5 \* \*

L7 \* \*

L5 \* \*

**Run 2: Time integration using path 30 and both landsats**

**Run 3: Time integration using path 29 and both landsats**

L7 \* \* \*

L7 \* \* \*

L7 \* \* \*

L7 \* \* \*

L7 \* \* \*

L5 \* \* \*

L5 \* \* \*

L5 \* \* \*

L5 \* \* \*

**Run 4: Time integration using path 29 and landsat L5**

**Run 5: Time integration using path 29 and landsat L7**

**Dates Sensor Run 1:** 

1 April 24, 2002

2 May 02, 2002

3 May 03, 2002

4 June 11, 2002

5 June 27, 2002

6 June 28, 2002

7 July 22, 2002

8 July 29, 2002

9 July 30, 2002

10 August 6, 2002

11 August 14, 2002

13 August 15, 2002

14 August 23, 2002

15 August 31, 2002

16 September 7, 2002

17 September 8, 2002

18 September 15, 2002

19 September 16, 2002

20 September 23, 2002

**56**

**Table 2.**

*platform.*

Three to four integration runs were made for the study areas, as described in the next section. The integration runs utilized (1) both Landsat 5 and 7 imagery from both paths; (2) both Landsat 5 and 7 imagery from one path or the other; and

*Selection of Landsat images used to time-integrate ET for the Nebraska study area, showing collection path and* 

*Asterisks indicate the dates used in a particular integration run to estimate monthly and growing season ET.*

## **2.4 Model runs**

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 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 speculation on the evolution and trends in ETrF over time.

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 only process imagery from Landsat 7 for path 29.
