ET Changes

ET Area

The sub-basin factors were adjusted during calibration, which resulted in some sub-basin factors not being equal to one for the period 1993-2000. Therefore, a new ET area file was generated which applies the 1993-2000 sub-basin factors to the original ET area file, resulting in a new ET area which sets the cell by cell ET area from 2001 onwards.

Maximum Monthly ET Rate

During the calibration phase and for the years 2001, 2002 and 2003, this calculation was performed by Spronk Water Engineers. The daily minimum and maximum temperature and precipitation were extracted from the National Climatic Data Center TD3200 summary of the day data sets. Missing climate data was filled in by average or interpolation. Then the Hargreaves ETo was calculated using the RefET program and transferred to a spreadsheet. This ETo was then adjusted to the Penman-Monteith ETr using TR-21 effective precipitation calculation and crop coefficients originally from Derrel Martin and modified by Mary-Kay Brengosz.

The end result was a monthly ET rate at each climate station which was then interpolated west-east over the domain and multiplied by the ET area to obtain the ET package input to the model.

The procedure applied until 2003 involved manually editing the climate data, importing it into RefET, exporting the data to a spreadsheet, and then exporting results from the spreadsheet. This procedure was time consuming.

Therefore, two new programs were developed. The program mkttp was developed to read climate data in TD3200 format downloaded from the National Climatic Data Center web site, perform data filling as necessary, and save the daily precipitation, minimum and maximum temperature for Akron, McCook and Red Cloud. The program mkmonet was developed to read these data, perform the ET calculations and save the monthly potential ET adjusted for precipitation at the same three stations.

The following table demonstrates the differences between the original method and the new method that will be applied from 2004 onwards.

Period	RefET+Spreadsheet			mkmonet			Difference
	Akron	McCook	Red Cloud	Akron	McCook	Red Cloud	Akron	McCook	Red Cloud
1/2001	0.36	0.43	0.48	0.44	0.44	0.53	0.08	0.01	0.05
2/2001	0.60	0.21	0.00	0.60	0.23	0.00	0.00	0.02	0.00
3/2001	1.00	0.70	0.82	1.01	0.73	0.84	0.01	0.03	0.02
4/2001	2.24	1.99	1.96	2.23	2.02	1.95	-0.01	0.03	-0.01
5/2001	4.89	5.41	3.14	4.90	5.47	3.14	0.01	0.06	0.00
6/2001	9.02	9.22	8.05	9.01	9.18	8.06	-0.01	-0.04	0.01
7/2001	9.69	7.91	5.22	9.63	7.86	5.17	-0.06	-0.05	-0.05
8/2001	8.95	8.45	7.75	8.95	8.49	7.72	0.00	0.04	-0.03
9/2001	6.36	4.91	4.53	6.46	4.96	4.58	0.10	0.05	0.05
10/2001	2.60	2.69	2.92	2.69	2.77	2.97	0.09	0.08	0.05
11/2001	1.11	0.81	1.16	1.11	0.82	1.19	0.00	0.01	0.03
12/2001	0.94	0.85	0.78	0.97	0.88	0.80	0.03	0.03	0.02
2001	47.76	43.58	36.81	48.00	43.85	36.95	0.24	0.27	0.14
1/2002	0.93	0.77	0.65	0.98	0.81	0.70	0.05	0.04	0.05
2/2002	1.12	0.99	0.50	1.12	1.02	0.51	0.00	0.03	0.01
3/2002	1.57	1.19	1.01	1.57	1.20	1.02	0.00	0.01	0.01
4/2002	4.90	4.08	3.19	4.89	4.08	3.18	-0.01	0.00	-0.01
5/2002	8.22	6.86	4.45	8.25	6.90	4.44	0.03	0.04	-0.01
6/2002	9.83	9.49	9.28	9.85	9.44	9.26	0.02	-0.05	-0.02
7/2002	12.61	10.25	9.80	12.52	10.20	9.77	-0.09	-0.05	-0.03
8/2002	8.04	7.54	6.40	8.02	7.54	6.37	-0.02	0.00	-0.03
9/2002	5.13	5.00	4.50	5.13	4.97	4.48	0.00	-0.03	-0.02
10/2002	1.51	0.93	0.44	1.54	0.78	0.46	0.03	-0.15	0.02
11/2002	0.95	1.06	1.23	0.95	1.08	1.26	0.00	0.02	0.03
12/2002	0.89	0.96	0.86	1.04	0.97	0.86	0.15	0.01	0.00
2002	55.70	49.12	42.31	55.86	48.99	42.31	0.16	-0.13	0.00
1/2003	0.94	0.91	0.63	1.02	0.97	0.69	0.08	0.06	0.06
2/2003	0.65	0.25	0.44	0.67	0.27	0.47	0.02	0.02	0.03
3/2003	0.61	1.55	1.17	0.62	1.52	1.19	0.01	-0.03	0.02
4/2003	3.39	3.05	2.05	3.49	3.14	2.07	0.10	0.09	0.02
5/2003	5.25	5.37	4.61	5.24	5.41	4.63	-0.01	0.04	0.02
6/2003	5.80	5.90	5.80	5.82	5.87	5.79	0.02	-0.03	-0.01
7/2003	11.92	8.68	10.63	11.87	9.68	10.58	-0.05	1.00	-0.05
8/2003	9.86	4.98	8.26	9.85	5.78	8.23	-0.01	0.80	-0.03
9/2003	6.58	5.38	4.12	6.63	5.37	4.13	0.05	-0.01	0.01
10/2003	3.24	2.63	3.43	3.26	2.69	3.44	0.02	0.06	0.01
11/2003	1.10	0.70	0.54	1.11	0.73	0.56	0.01	0.03	0.02
12/2003	1.00	0.84	0.75	1.00	0.85	0.77	0.00	0.01	0.02
2003	50.34	40.24	42.43	50.58	42.28	42.55	0.24	2.04	0.12
Average	51.27	44.31	40.52	51.48	45.04	40.60	0.21	0.73	0.09

The differences between the previous calculations and the new method occurred because of two reasons.

1. The filling of missing temperature or precipitation data was performed manually before. The new procedure uses a systematic algorithm to perform filling and interpolation of the climate data, which may result in slightly different values being substituted for missing data in the two methods.
2. The transfer of data between programs, primarily between the RefET program and the spreadsheet, was done using text files, which resulted in daily ET values being rounded. In the new procedure, all the calculations are done in the same program with no rounding involved, which leads to slightly different monthly total values.

While the new method does not yield exactly the same results as before, the differences are not significant. However, it should be noted that the new procedure will only be used to calculate the monthly maximum ET values for Akron, McCook and Red Cloud starting in 2004, while the previous procedure was used until 2003.