**3.4 Statistical summaries**

*Advanced Evapotranspiration Methods and Applications*

**66**

**Figure 15.**

*runs.*

**Figure 14.**

the baseline run 1 by about 19% on average.

Growing season (March–September) ET produced by the time-integration is plotted in **Figure 18** for the Nebraska study area for runs 3, 2, and 5 versus run 1. Agreement was strongest between baseline run 1 and run 2 that used images from both Landsats from path 29 only. Growing season ET produced from path 29 using only Landsat 7 images only had the worse correlation with r2 = 0.64. Growing season ET produced from path 29 images, tended to overestimate ET according to

*Monthly ET averaged from the 1500 sample pixels for the Nebraska study area for the five time-integration* 

*Plots of average integrated ETrF for August from model run 2 (two Landsats on path 29), run 3 (two satellites on path 30), run 4 (Landsat 5, only on path 29), and run 5 (Landsat 7, only on path 29) versus ETrF produced* 

*from the baseline model run 1 for the central Nebraska analysis area for year 2002.*

**Table 3** summarizes monthly average ET for the four time-integration runs for the Idaho study area and root mean square error (RMSE) for the 1500 sampled fields. RMSE was relatively high for run 2 (both satellites for path 39 only),

**Figure 16.**

*Monthly ETrF averaged over the 1500 sample pixels for the Nebraska study area for the five time-integration runs.*

**Figure 17.**

*ET for April–October growing season for 1500 sampled locations for the Idaho study area for the timeintegration runs 2, 3, and 4 versus run 1 and (lower right) averages over all 1500 sampled fields. Also shown in the lower right is reference ET summed over the April–October period.*

#### **Figure 18.**

*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).*


#### **Table 3.**

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

**69**

*\**

**Table 5.**

**Table 4.**

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

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

Both paths 849 0.00 0.60 Path 39 778 8.30 0.50 Path 40 838 1.25 0.53 Path 40\_L5 834 1.74 0.53

*Growing season ET (April–October 2000) averaged over 1500 sampled fields in the Idaho study area for the four* 

**Average** Both paths 34 159 194 156 123

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

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

**ETr (mm)** 184 259 232 173 158

*Average monthly ET, RMSE, percentage error, and total monthly reference ET for the 1500 pixel sample* 

Path 30 130 169 210 152 123 Path 29 90 178 199 153 120 Path 29\_L5 only 59 164 197 151 123 Path 29\_L7 only 94 197 205 164 93

Path 30 104 19 31 10 6 Path 29 61 45 8 6 7 Path 29\_L5 only 52 45 14 8 8 Path 29\_L7 only 64 63 19 14 32

Path 30 56 7 13 6 4 Path 29 33 17 3 3 4 Path 29\_L5 only 28 17 6 5 5 Path 29\_L7 only 35 24 8 8 29

**ET (mm) % Diff ETrF**

**Average ET, RMSE, and Total ETr are in mm—Year 2002 May June July August Sept**

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

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

ETr 1422

*runs and percent differences from the base run 1.*

*Percentage error is RMSE error relative to ETr.*

*locations for the Nebraska study area.*
