The measurements needed for the Keetch-Byram Drought Index are (1) the maximum air temperature (or the dry-bulb temperature at the time of the basic fire weather observation), (2) the total rainfall for the past 24 hours, and (3) the appropriate drought factor table. Drought factor tables are based on mean annual rainfall and the rate of moisture loss from soil, which is determined by known evapotranspiration processes related to the relationship between the type and density of vegetation and the amount of annual rainfall. A Drought Index may be computed for any desired level of mean annual rainfall. However, to simplify the computations for field use, five tables of drought factors were constructed. Each table covers a specific range of mean annual rainfall. Only one drought-factor table is needed for each fire weather station.

Example drought factor table.....

DROUGHT FACTOR TABLE FOR ANNUAL RAINFALL 40 TO 59 INCHES

TEMP	DROUGHT INDEX YESTERDAY
	0 49	50 99	100 149	150 199	200 249	250 299	300 349	350 399	400 449	450 499	500 549	550 639	640 699	700 759	760 799	800

107+	62	58	54	50	46	42	38	34	30	26	22	16	10	6	2	0
104-106	53	50	46	43	39	36	33	29	26	22	19	14	8	5	1	0
101-103	46	43	40	37	34	31	28	25	22	19	16	12	7	4	1	0
98-100	39	37	34	31	29	26	24	21	19	16	14	10	6	4	1	0
95-97	33	31	29	27	25	23	20	18	16	14	12	9	5	3	1	0
92-94	28	27	25	23	21	19	17	16	14	12	10	8	4	3	1	0
89-91	24	23	21	20	18	16	15	13	12	10	9	6	4	2	1	0
86-88	21	19	18	17	15	14	13	11	10	9	7	5	3	2	1	0
.
.
.
77-79	12	11	11	10	9	8	8	7	6	5	4	3	2	1	1	0
74-76	10	10	9	8	8	7	6	6	5	4	4	3	2	1	1	0
.
68-70	7	6	6	6	5	5	4	4	3	3	2	2	1	1	0	0
65-67	6	5	5	4	4	4	3	3	3	2	2	2	1	1	0	0
.
.
.
50-52	1	1	1	1	1	1	1	1	1	1	1	1	0	0	0	0

Starting a Drought Index Record

From an examination of the drought factor table shown above, it becomes apparent that for any given temperature, the drought factor to be added each day depends on the Drought Index from the previous day. This cumulative feature means that a Drought Index record for a particular station cannot automatically start at zero. This is especially true for fire weather stations that begin seasonal observations later in the spring, at a time when the soils may no longer be saturated from winter snowmelt or spring rains. The zero point may have occurred weeks or months before, or even during the previous year. In these situations, it is necessary to go back in time unit a day is reached when it is reasonably certain that the upper soil layers are saturated, then bring the record forward day by day to the actual starting time of the fire weather observations. In mountainous areas in the west, it is usually safe to assume saturation occurs just after the spring snow melt, or after a period of abundant rainfall, say 6 to 8 inches in a period of a week.

When the starting point has been determined and the proper drought factor table has been selected, the computation of the daily Drought Index is a simple bookkeeping procedure. The two steps involved are:

Reduce the drought index by the amount of net rain, if any.
Increase the drought index by the amount found in the drought factor table

DROUGHT INDEX SAMPLE RECORD




DAY	24HR PCP	NET PCP	AIR TEMP	DROUGHT INDEX (YESTERDAY)	DROUGHT FACTOR	DROUGHT INDEX TODAY	CURRENT DROUGHT STAGE

1		0	79	164	10	174	1
2		0	75	174	8	182	1
3	0.66	0.46	70	136	6	142	1
4	T	0	76	142	9	151	1
5	0.23	0.03	79	148	11	159	1
6		0	84	159	14	173	1
7	0.16	0	65	173	4	177	1
8	0.09	0.05	66	172	4	176	1
9		0	83	176	14	190	1
10		0	70	190	6	196	1
11	0.08	0	67	196	4	200	2
12	0.03	0	65	200	4	204	2
13		0	76	204	8	212	2
14	0.22	.02	69	210	5	215	2
15		0	65	215	4	219	2
16	0.21	.01	75	218	8	226	2
17		0	78	226	9	235	2
18		0	85	235	13	248	2
19		0	88	248	15	263	2
20	0.01	0	79	263	8	271	2
21		0	69	271	5	276	2
22		0	75	276	7	283	2
23		0	84	283	12	295	2
24		0	89	295	16	311	3
25		0	93	311	17	328	3
26		0	92	328	17	345	3
27		0	96	345	20	365	3
28		0	91	365	13	378	3
29	0.25	.05	78	373	7	380	3
30	0.16	.16	83	364	10	374	3

Explanation of the drought index sample record

Col. 1 - day of the month

Col. 2 - measured amount of rain in the past 24 hours to the nearest 0.01 inch

Col. 3 - subtract 0.20 from the amount in column 2 to obtain the net rainfall. If there are consecutive rainy days with no drying of the tree canopy between showers, subtract 0.20 only once, on the day that the cumulative rainfall exceeds 0.20. inches. Thereafter, consider all of the rain in col. 2 as net rainfall and transfer that amount to col. 3 until the wet spell ends. the wet spell ends when there is a 24 hour period with no rainfall.

Example 1 - 0.20 was subtracted from each of the individual rains on the 3rd, 5th, 14th, and 16th.

Example 2 - Rain on the 7th and 8th was consecutive, so the 0.20 was subtracted on the 8th when the total rain exceeded 0.20 inches.

Example 3 - Rain on the 29th and 30th was consecutive, so 0.20 was subtracted on the 29th, and all of the rain on the 30th was transferred to col. 3.

Col. 4 - air temperature, either maximum for the day, or the dry-bulb temperature at the time of the basic observation.

Col. 5 - Yesterday's Drought Index, or as reduced by the net rainfall in col. 3.

Example 1 - No net rain on the 2nd. Drought Index was 174 on the first. Therefore, carry 174 forward to column 5 for the 2nd.

Example 2 - Net rain on the 3rd was 0.46. Drought Index on the 2nd was 182 (col. 7). Therefore, 182 minus 46 equals 136, which is recorded in col. 5 for the 3rd.

Col. 6 - Use the appropriate Drought Factor Table (shown above) to determine the today's drought factor based on the temperature from col. 3 and the Drought Index from col. 5.

Col. 7 - Add yesterday's Drought Index from col. 5 to the drought factor in col. 6 to obtain today's Drought Index.

Col. 8 - The current stage of drought. The incipient stage (0) includes KBDI values from 0 to 99. The first stage (1) includes KBDI values from 100 to 199, second stage 200 to 299, and so on through the seventh stage from 700 to 800.

One way to emphasize the importance of selecting the proper drought factor table is to compute, according to each of the five drought factor tables, the number of consecutive days having a constant maximum temperature and no rainfall that must elapse (after starting a zero) before a selected stage of drought is reached. The following tabulation lists the number of days required for each of the five tables to reach the fifth stage of drought (500), when the observed temperature each day ranges from 80 to 82.

Table Number	Mean Annual Rainfall	Consecutive number of drying days needed to reach D.I. 500

1	10-19	157

2	20-29	109

3	30-39	78

4	40-59	52

5	60 or more	36

We can visualize the two extremes represented by the drought factor tables. If the two areas represented by tables 1 and 5 are both started with zero drought Index on May 31, then the area with heavy rainfall (table 5) would reach stage 5 drought in 36 consecutive days (July 6), and the area of light precipitation (table 1) would reach stage 5 in 157 consecutive days (November 4).

Please refer any questions or comments about this web site to: john.werth@noaa.gov