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

exceeding 20% of reference ET for 5 of 7 months. RMSE for runs 3 and 4 had less error than run 2 for 2 months, even though run 4 utilized only seven image dates (from Landsat 5 and path 40) in the integration. This indicates the importance of timing of images to identify key inflection points in the ETrF curves and to capture special events such as wetting events from irrigation and rain or from water stress or cuttings, as in the case of alfalfa hay. **Table 4** summarizes growing season comparisons for ET among the four runs. The runs that used images from path 40 only compared to within 2%, when averaged over all 1500 fields, to the baseline run. This outcome is likely due to the timing of path 40 images relative to the combined run as compared to path 39. This shows the high value of a high density of image dates so that important inflection points in ETrF curves can be obtained.

**Table 5** provides monthly average ET, RMSE, percentage error (RMSE error in relation to ETr), and total monthly reference ET for the 1500 pixel sample locations in the Nebraska study area. RMSE was high for all model runs for May and June, exceeding 19%, with a maximum RMSE of 104% for model run 2 for May. Model run 2 (both Landsats from path 30 only) had lower error for June compared to May


#### **Table 4.**

*Advanced Evapotranspiration Methods and Applications*

*Total ET for May–September growing season of 2002 for 1500 sampled locations for the Nebraska study area for time-integration model runs 3, 2, and 5 versus model run 1. Also shown in the lower right is total growing season ET averaged over all samples and reference ET from two AWDN stations (right two columns).*

**Average** Both paths 40 99 173 225 159 99 54

**RMSE** Both paths 0 0 0 0 0 0 0

**% error\*** Both paths 0 0 0 0 0 0 0

ETr 174 200 274 265 227 171 111

*Average monthly ET over the 1500 sampled fields in the Idaho study area for the four runs and RMSE,* 

**Average ET, RMSE, and Total ETr are in mm—Year 2000**

Path 39 35 79 165 204 153 98 43 Path 40 33 94 187 233 165 80 46

Path 39 34 48 18 76 51 19 23 Path 40 19 41 23 55 41 35 12

Path 39 20 24 7 29 23 11 21 Path 40 11 21 9 21 18 21 11

**April May June July August Sept Oct**

37 93 187 233 167 84 33

25 40 23 54 43 37 27

14 20 9 21 19 22 24

**68**

*\**

**Table 3.**

**Figure 18.**

Path 40\_L5 only

Path 40\_L5 only

Path 40\_L5 only

*% error is RMSE error relative to ETr.*

*percentage error and reference ET.*

*Growing season ET (April–October 2000) averaged over 1500 sampled fields in the Idaho study area for the four runs and percent differences from the base run 1.*


#### **Table 5.**

*Average monthly ET, RMSE, percentage error, and total monthly reference ET for the 1500 pixel sample locations for the Nebraska study area.*

or July, which emphasizes the impact of the timing of the images used. For path 30, image date June 28, 2002 had large areas of clouds masked out, which were filled in using the next available image date in time in the spline function. This underscores the importance of timing of images to identify key inflection points in the ETrF curves and to capture special events such as wetting events from irrigation and rain or from water stress or cuttings, as in the case of alfalfa hay.
