Step-by-Step Methodology for Dry Spell Analysis

1. Data Preparation

• Collect daily rainfall data for study location(s) for the defined period (e.g., baseline: 1991–2020). • Define the rainy season period (i.e., period between start and end of season).

2. Define Dry Spell Threshold

• A dry spell is typically defined as a sequence of consecutive days with rainfall below a threshold. The threshold we used is: o 1.0 mm/day (widely used for agricultural dry spells) o 7 consecutive dry days (often used in agricultural and climate studies)

o Adopted definition: The dry spell was defined as a period of at least 7 consecutive days with daily rainfall below 1.0 mm.

3. Identify Dry Spells

• For each year, scan the daily rainfall data within the rainy season. • Identify sequences of consecutive days where rainfall is below the chosen threshold. • Record the start date, end date, and length (number of days) of each dry spell.

4. Calculate Dry Spell Statistics

• For each year, compute: o Number of dry spells o Maximum sequence of dry spell o Mean dry spell length o Total number of dry days • For the entire period (1991–2020), compute: o Average number of dry spells per year o Average maximum sequence of dry spell o Average mean dry spell length o Frequency distribution of dry spell lengths

Key Indicators to Display

Indicator	Description
Number of dry spells/year	Total number of dry spells identified in each year within the rainy season
Maximum sequence of dry spell	Longest dry spell (in days) in each year within the rainy season
Mean dry spell length	Average length of all dry spells in each year within the rainy season
Total dry days/year	Total number of days classified as dry in each year within the rainy season
Frequency distribution	Distribution of dry spell lengths (e.g., 7–10 days, 11–15 days, etc.)

5. Present Results: Table

Year	Spell length (days)	Number of dry spells/year	Maximum sequence of dry spell	Mean dry spell length	Total dry days/year

A flow diagram

Reference

1. G.S. Pratyusha Kranthi, K.V. Rao, K. Padmakumari. Dryspell analysis using gridded rainfall data for Telangana. Agriculture Association of Textile Chemical and Critical Reviews Journal (2024) 71-75.
2. J. Barrona, J. Rockströmb, F. Gichukic, N. Hatibud. Dry spell analysis and maize yields for two semi-arid locations in East Africa. Agricultural and Forest Meteorology 117 (2003) 23–37.
3. R. D. Stern, M. D. Dennett, I. C. Dale. Analysing daily rainfall measurements to give agronomically useful results. II A modelling approach. Expl Agric. (1982), volume 18, pp. 237-253.
4. R. D. Stern, M. D. Dennett, I. C. Dale. Analysing daily rainfall measurements to give agronomically useful results. I Direct methods. Expl Agric. (1982), volume 18, pp. 223-236.
5. Caribbean Regional Climate Center. Dry spells outlook