**2. Methods and materials**

### **2.1 Study areas**

Two application areas were utilized for this test. One area was in southern Idaho and the second area was in central Nebraska. Idaho is a relatively 'clear' area, so that, this analysis represents a somewhat 'optimistic' scenario as compared to more cloud-prone parts of the USA, for example, the Midwestern states. Central Nebraska has relatively high amounts of cloud cover and presents a greater challenge in obtaining a sufficient temporal density of clear imagery to produce accurate time-integrated estimates of ET.

In both areas, a subarea of Landsat images located in a WRS path overlap was selected for study. In Idaho, the subarea resided within path 39 row 30 and path 40 row 30. The area is shown in **Figure 2**, where the dimensions of the study area were approximately 50 km east–west × 80 km north–south. The study area contained a mixture of irrigated agriculture comprised of potatoes, sugar beets, alfalfa, peas, dry beans, corn, small grains (wheat and barley), and pasture surrounded by areas of sagebrush desert with some grasslands. The upper part of the study area contains basaltic flows from the Craters of the Moon National Monument and some mountainous terrain lies to the south. Annual precipitation

#### **Figure 2.**

*Idaho study area, outlined in blue, that lies in the overlap of Landsat WRS paths 40 (on the left) and 39 (on the right) overlaid onto a false color composite from Landsat 7 on August 14, 2000 for path 40 and Landsat 5 on June 28, 2000 for path 39. Irrigated areas along the Snake River plain are shown as bright reds.*

**53**

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

is about 200 mm and all agricultural crops are irrigated from surface water or

*Landsat 7 on July 29, 2002 for path 29 and Landsat 7 on July 22, 2002 for path 30.*

*Nebraska study area shaded in yellow where path 29, rows 30–31 and path 30, rows 30–31 overlap. The area managed by Central Platte NRD is outlined in white. The images shown are false color composites from* 

In Nebraska, the subarea resided within the domain of the Central Platte Natural Resources District (NRD) that lies within the overlap of WRS path 30 rows 30–31 and path 29 rows 30–31. That study area in central Nebraska is shown in **Figure 3** and has dimensions of approximately 60 km by 50 km with an area

crops grown in this section of Nebraska, with some alfalfa cultivation as well. Agricultural irrigation is important to this area with over 18,000 irrigation wells and over 1 million certified irrigated acres. The dominant irrigation method in the area is center pivot and the irrigation season generally lasts from mid-June to mid-September. Annual precipitation for this area is approximately 600–650 mm. **Figure 4** shows a close-up of the Nebraska study area showing the distribution of

**Table 1** lists the selection of Landsat images used to time-integrate ET in the Idaho study area. Dates for both path 39 and 40 are listed as well as the Landsat platform that collected the images. Year 2000 was selected for the analysis because it was during a 'golden period' of Landsat imagery, where two fully functioning satellites were in operation. Year 2000 was also a year that had previously been processed using METRIC so that those results were available for use in this analysis. Asterisks in **Table 1** indicate the dates used in a particular integration run to estimate monthly and growing season ET. The application of METRIC to the two paths for year 2000 is described in Allen et al. [12] and Trezza [8]. Most of the images listed in **Table 1** were essentially clear images for the small study area and did not require mitigation for clouds. The exception was August 23, 2000 that was half-cloud covered. That image cloud mask was used with the spline model to signal the need to expand the spline to an additional image date. There were a few clear images for the study area

. Corn and soybeans are the predominant agricultural

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

from ground water.

**Figure 3.**

irrigated fields.

**2.2 Remote sensing data**

of approximately 1900 km<sup>2</sup>

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

#### **Figure 3.**

*Advanced Evapotranspiration Methods and Applications*

**2. Methods and materials**

time-integrated estimates of ET.

**2.1 Study areas**

*Idaho in year 2000.*

**Figure 1.**

**52**

**Figure 2.**

*Idaho study area, outlined in blue, that lies in the overlap of Landsat WRS paths 40 (on the left) and 39 (on the right) overlaid onto a false color composite from Landsat 7 on August 14, 2000 for path 40 and Landsat 5* 

Two application areas were utilized for this test. One area was in southern Idaho and the second area was in central Nebraska. Idaho is a relatively 'clear' area, so that, this analysis represents a somewhat 'optimistic' scenario as compared to more cloud-prone parts of the USA, for example, the Midwestern states. Central Nebraska has relatively high amounts of cloud cover and presents a greater challenge in obtaining a sufficient temporal density of clear imagery to produce accurate

*Constructed Kc (or ETrF) curve for a bean crop from METRIC (dark symbols) with comparison against a standard Kc curve produced by the US Bureau of Reclamation Agrimet service for a region near Twin Falls,* 

In both areas, a subarea of Landsat images located in a WRS path overlap was selected for study. In Idaho, the subarea resided within path 39 row 30 and path 40 row 30. The area is shown in **Figure 2**, where the dimensions of the study area were approximately 50 km east–west × 80 km north–south. The study area contained a mixture of irrigated agriculture comprised of potatoes, sugar beets, alfalfa, peas, dry beans, corn, small grains (wheat and barley), and pasture surrounded by areas of sagebrush desert with some grasslands. The upper part of the study area contains basaltic flows from the Craters of the Moon National Monument and some mountainous terrain lies to the south. Annual precipitation

*on June 28, 2000 for path 39. Irrigated areas along the Snake River plain are shown as bright reds.*

*Nebraska study area shaded in yellow where path 29, rows 30–31 and path 30, rows 30–31 overlap. The area managed by Central Platte NRD is outlined in white. The images shown are false color composites from Landsat 7 on July 29, 2002 for path 29 and Landsat 7 on July 22, 2002 for path 30.*

is about 200 mm and all agricultural crops are irrigated from surface water or from ground water.

In Nebraska, the subarea resided within the domain of the Central Platte Natural Resources District (NRD) that lies within the overlap of WRS path 30 rows 30–31 and path 29 rows 30–31. That study area in central Nebraska is shown in **Figure 3** and has dimensions of approximately 60 km by 50 km with an area of approximately 1900 km<sup>2</sup> . Corn and soybeans are the predominant agricultural crops grown in this section of Nebraska, with some alfalfa cultivation as well. Agricultural irrigation is important to this area with over 18,000 irrigation wells and over 1 million certified irrigated acres. The dominant irrigation method in the area is center pivot and the irrigation season generally lasts from mid-June to mid-September. Annual precipitation for this area is approximately 600–650 mm. **Figure 4** shows a close-up of the Nebraska study area showing the distribution of irrigated fields.

### **2.2 Remote sensing data**

**Table 1** lists the selection of Landsat images used to time-integrate ET in the Idaho study area. Dates for both path 39 and 40 are listed as well as the Landsat platform that collected the images. Year 2000 was selected for the analysis because it was during a 'golden period' of Landsat imagery, where two fully functioning satellites were in operation. Year 2000 was also a year that had previously been processed using METRIC so that those results were available for use in this analysis. Asterisks in **Table 1** indicate the dates used in a particular integration run to estimate monthly and growing season ET. The application of METRIC to the two paths for year 2000 is described in Allen et al. [12] and Trezza [8]. Most of the images listed in **Table 1** were essentially clear images for the small study area and did not require mitigation for clouds. The exception was August 23, 2000 that was half-cloud covered. That image cloud mask was used with the spline model to signal the need to expand the spline to an additional image date. There were a few clear images for the study area

#### **Figure 4.**

*Close-up view of the Nebraska study area extent. The white line is the Central Platte NRD boundary. Bright red areas are cultivated fields and the lighter areas are rangeland. The areas of high densities of fields are irrigated areas along the Platte River, which is visible along the southern boundary of the Central Platte NRD. Those fields utilize a combination of ground water and surface water. Areas of more sparse densities of irrigated fields are fields using primarily ground water a water source.*

that had not been processed by METRIC due to the close coincidence of other clear images in time. An example is June 28, 2000 for path 39, which was not processed. Therefore, the list of images in **Table 1** is not all inclusive. However, the absence of images should not impact the accuracy of the baseline estimation of timeintegrated ET because there are sufficient data points to afford a relatively accurate interpolation.

**Table 2** lists the selection of Landsat images used to time-integrate ET in the Nebraska study area. As with the Idaho study area, year 2002 was selected for analysis because it was a year when both Landsat 5 and 7 satellites were in operation and fully functioning. Asterisks in **Table 2** indicate dates that they were used in a particular integration run to estimate monthly and growing season ET.

Most of the imagery listed were clear images for the small study area and did not require mitigation for clouds. The exceptions were June 27, 2002 and August 6, 2002 from path 30, with both images having significant cloud cover over the study area. Cloud-covered areas in the imagery were masked out by manually tracing around the cloud areas and filling those cloud areas with a value recognizable in the timeintegration models as invalid. The masked out areas were replaced with data from a previous or following image date during the spline function in the time-integration.
