Measuring Vapor Pressure Deficit in the Field

R.L. Snyder and K.T. Paw U                                                                                         
Copyright – Regents of the University of California

Created – January 6, 2002

Last Update – Revised April 2006

Copyright 2002 – Regents of the University of California

This paper presents a simple method to determine the vapor pressure deficit (VPD) in the field.  The VPD is often used to determine baseline curves for plant-based measures of water status (e.g., with infrared thermometers or with stem water potential).  Both the air (T) and dew point (T_d) temperature are measured.  Then the saturation vapor pressure is calculated using the equation

The actual vapor pressure (e) is calculated using the same equation, but with the dew point temperature (T_d) substituted for T in the equation.  Rather than making the calculations, the following two tables can be used to determine e_s and e from T and T_d.  The two tables are provided so one can print them front to back on a single sheet of paper.  The paper can be laminated to prevent damage when working in the field. The first table is for temperatures 32^oF through 79^oF and the second has temperatures 80^oF through 120^oF.  To use the table, find the dew point temperature

				Vapor Pressure in KPa
Temp (^oF)	0	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
32	0.61	0.61	0.62	0.62	0.62	0.62	0.63	0.63	0.63	0.63
33	0.64	0.64	0.64	0.64	0.65	0.65	0.65	0.65	0.66	0.66
34	0.66	0.66	0.67	0.67	0.67	0.68	0.68	0.68	0.68	0.69
35	0.69	0.69	0.69	0.70	0.70	0.70	0.71	0.71	0.71	0.71
36	0.72	0.72	0.72	0.73	0.73	0.73	0.73	0.74	0.74	0.74
37	0.75	0.75	0.75	0.75	0.76	0.76	0.76	0.77	0.77	0.77
38	0.78	0.78	0.78	0.79	0.79	0.79	0.79	0.80	0.80	0.80
39	0.81	0.81	0.81	0.82	0.82	0.82	0.83	0.83	0.83	0.84
40	0.84	0.84	0.85	0.85	0.85	0.86	0.86	0.86	0.87	0.87
41	0.87	0.88	0.88	0.88	0.89	0.89	0.89	0.90	0.90	0.90
42	0.91	0.91	0.91	0.92	0.92	0.92	0.93	0.93	0.94	0.94
43	0.94	0.95	0.95	0.95	0.96	0.96	0.96	0.97	0.97	0.98
44	0.98	0.98	0.99	0.99	0.99	1.00	1.00	1.01	1.01	1.01
45	1.02	1.02	1.03	1.03	1.03	1.04	1.04	1.04	1.05	1.05
46	1.06	1.06	1.06	1.07	1.07	1.08	1.08	1.09	1.09	1.09
47	1.10	1.10	1.11	1.11	1.11	1.12	1.12	1.13	1.13	1.14
48	1.14	1.14	1.15	1.15	1.16	1.16	1.17	1.17	1.17	1.18
49	1.18	1.19	1.19	1.20	1.20	1.21	1.21	1.21	1.22	1.22
50	1.23	1.23	1.24	1.24	1.25	1.25	1.26	1.26	1.27	1.27
51	1.27	1.28	1.28	1.29	1.29	1.30	1.30	1.31	1.31	1.32
52	1.32	1.33	1.33	1.34	1.34	1.35	1.35	1.36	1.36	1.37
53	1.37	1.38	1.38	1.39	1.39	1.40	1.40	1.41	1.41	1.42
54	1.42	1.43	1.43	1.44	1.44	1.45	1.45	1.46	1.47	1.47
55	1.48	1.48	1.49	1.49	1.50	1.50	1.51	1.51	1.52	1.53
56	1.53	1.54	1.54	1.55	1.55	1.56	1.56	1.57	1.58	1.58
57	1.59	1.59	1.60	1.60	1.61	1.62	1.62	1.63	1.63	1.64
58	1.65	1.65	1.66	1.66	1.67	1.68	1.68	1.69	1.69	1.70
59	1.71	1.71	1.72	1.72	1.73	1.74	1.74	1.75	1.75	1.76
60	1.77	1.77	1.78	1.79	1.79	1.80	1.81	1.81	1.82	1.82
61	1.83	1.84	1.84	1.85	1.86	1.86	1.87	1.88	1.88	1.89
62	1.90	1.90	1.91	1.92	1.92	1.93	1.94	1.94	1.95	1.96
63	1.97	1.97	1.98	1.99	1.99	2.00	2.01	2.01	2.02	2.03
64	2.04	2.04	2.05	2.06	2.06	2.07	2.08	2.09	2.09	2.10
65	2.11	2.12	2.12	2.13	2.14	2.14	2.15	2.16	2.17	2.17
66	2.18	2.19	2.20	2.21	2.21	2.22	2.23	2.24	2.24	2.25
67	2.26	2.27	2.27	2.28	2.29	2.30	2.31	2.31	2.32	2.33
68	2.34	2.35	2.35	2.36	2.37	2.38	2.39	2.40	2.40	2.41
69	2.42	2.43	2.44	2.44	2.45	2.46	2.47	2.48	2.49	2.50
70	2.50	2.51	2.52	2.53	2.54	2.55	2.56	2.56	2.57	2.58
71	2.59	2.60	2.61	2.62	2.63	2.63	2.64	2.65	2.66	2.67
72	2.68	2.69	2.70	2.71	2.72	2.73	2.73	2.74	2.75	2.76
73	2.77	2.78	2.79	2.80	2.81	2.82	2.83	2.84	2.85	2.86
74	2.87	2.88	2.89	2.90	2.91	2.91	2.92	2.93	2.94	2.95
75	2.96	2.97	2.98	2.99	3.00	3.01	3.02	3.03	3.04	3.05
76	3.06	3.07	3.08	3.10	3.11	3.12	3.13	3.14	3.15	3.16
77	3.17	3.18	3.19	3.20	3.21	3.22	3.23	3.24	3.25	3.26
78