Understanding your seasonal rainfall pattern and what it means for strategy
BY THE END OF THIS ARTICLE, YOU WILL BE ABLE TO…
- Identify your farm’s dry season length and timing,
- Map the areas of your farm with the highest evaporative demand, and
- Locate your climate pattern in the strategy table to identify where to focus first.
SHARED VOCABULARY
Climate: The long-term pattern of weather at a location, characterized over 30 or more years. Includes seasonal averages, typical ranges, and the degree of year-to-year variability.
Weather: The specific conditions of a given day, week, or season. Weather varies within the bounds your climate establishes what you respond to; climate is what you design for.
Climograph: A chart showing average monthly temperature and precipitation over a reference period (typically 1991–2020). The fastest way to read your seasonal water pattern and identify your dry window.
Reference evapotranspiration (ET₀): A standardized estimate of how much water the atmosphere draws from the landscape through evaporation and transpiration. Increases with higher temperatures, stronger wind, lower humidity, and more solar radiation. Helps you understand how quickly stored soil moisture is depleted, independent of whether rain has fallen.
Microclimate: Local conditions within a specific field, bed, or slope that differ from the regional average. Shaped by topography, aspect, wind exposure, shade, and vegetation. Some variation is fixed; some can be modified through farm design.
Climate is the entry point to this work because while you can’t choose your climate, you can create and take advantage of favorable microclimates to reduce water loss on your farm. What matters most is not only your total annual rainfall but how much falls and when.
The Historical Climograph is a tool that shows average monthly temperature and precipitation over a reference period. This tool can help provide a snapshot of the climate context you are working with. For example, in the climograph below, you can see that in Corvallis, OR, less than 3 inches of precipitation on average falls during June through September, the dry season, which coincides with the highest annual temperatures, averaging 73 to 83 degrees F. More than 38 inches of precipitation falls from October through May, which is enough to saturate the entirety of the soil profile and provide ample soil moisture for spring planting. High temperatures combined with low precipitation create the conditions where water-resilience strategies matter most.
ACTIVITY 1 – Look up your climograph
To get a snapshot of the climate context you are working with, go to climatetoolbox.org and generate a Historical Climograph for your location using the 1991–2020 reference period.
Using your climograph, answer the following questions:
- What months show the highest average precipitation? What months show the lowest?
- How many consecutive months average less than 1 inch of rain?
- Does the driest period overlap with the hottest temperatures?
- What crops are you growing when temperatures are high and rainfall is low?
- What crops can you grow outside of that pattern?
- Does anything surprise you, or confirm previous observations?
| Notes: |
ACTIVITY 2 – Map evapotranspiration and microclimate pressures on your farm
Reference evapotranspiration (ET₀) is a standardized estimate of the amount of water the atmosphere draws from the landscape through evaporation and transpiration. ET increases with higher temperatures, stronger winds, lower humidity, and more solar radiation. Knowing ET helps you understand how quickly stored soil moisture is depleted, independent of whether rain has fallen. For field-level ET data, see OpenET or AgriMet (PNW only) in the resources box below.
For each distinct area of your farm, note site characteristics that influence how quickly soil moisture is depleted.
| Location/area | Exposure (wind, sun, slope) | Observation | Notes / what you’ve tried to reduce evapotranspiration or alter your microclimate |
|---|---|---|---|
| Example: West side of the field (east of cover crop) | Sun and wind exposure | Early peppers got sunburnt | As the summer cover crop grew taller, it served as a windbreak and prevented sunburn for later fruit |
Questions worth working through:
- Where on your farm is ET pressure likely to be the highest. Take note of the spots that dry out fastest, receive the most wind, or heat up earliest.
- Where could a windbreak, shade structure, or ground cover meaningfully reduce atmospheric demand on crops?
ACTIVITY – ACTIVITY 3 – Map your last 5–7 seasons
Using farm records, journals, or memory, characterize each recent growing season. The goal is to start recognizing patterns and understanding how the local and regional climate influenced what was happening in your field and/or farm business.
If you don’t have personal data to draw from, The National Oceanic and Atmospheric Administration (NOAA)’s historical climate data tool may provide valuable insights. You could also talk with nearby growers and/or local resource providers — they may have some insights to share about the history of the land you’re growing on.
| Year | Observations | What made it challenging? | What mitigation or management strategies would you implement moving forward? |
|---|---|---|---|
| Example: 2023 | Example: long, wet spring | Example: Plant start business suffered | Example: Diversify early-season offerings |
After completing the table, consider:
- What patterns do you notice across seasons? Consider timing, intensity, or the challenges those patterns created?
- Which conditions are becoming more frequent or more severe, and how does that inform what you want to be prepared for?
SEE IT IN CONTEXT – Case Studies
The case studies in Section 4 show how growers in specific contexts have applied the tools in this article. They’re intended to be illustrative examples. You may not see your climate, your scale, or your crops reflected in what’s currently available. Each farm is unique. If your context isn’t reflected yet, your experience is what this community needs. We’re actively building more examples. Email us at info@dryfarming.org to learn more.
From pattern to strategy
Match the row or rows that most closely describe your farm. Most growers will recognize themselves in more than one. Check the box next to each one that applies. Use this information as a starting point, not a prescription.
| If your climate pattern looks like this… | ✓ | Strategy areas worth exploring first |
|---|---|---|
| Reliable wet season followed by a long, predictable dry season (3–5+ consecutive months below 1 inch) | □ | Dry farming trials; deep soil building for water storage; planting timing and variety selection matched to the dry window |
| Dry season present but shorter or more variable — some years severe, others much less so | □ | Flexible irrigation management; soil moisture monitoring to guide decisions; cover crops and mulch to extend soil moisture; drought-tolerant variety exploration |
| High year-to-year variability — difficult to predict what the season will bring | □ | Practices robust across a range of conditions: organic matter building, diverse cover cropping, water storage, monitoring tools, crop and variety diversification |
| High evaporative demand throughout the growing season — hot, windy, exposed site | □ | Windbreaks and shelterbelts; shade structures or strategic interplanting; mulch; early-morning irrigation timing; reduced planting density in highest-exposure areas |
| Seasonal flooding or waterlogging in wet months alongside dry-season stress | □ | Drainage and soil structure management alongside water retention, earthworks, contour strip cropping, and organic matter to improve both drainage and moisture retention in dry conditions |
PUTTING IT INTO PRACTICE
- Which row in the table most closely describes your farm’s dry season? Write it down or check the box.
- What is your dry window? Note the specific months when rainfall reliably drops below 1 inch.
- Which area of your farm has the highest ET pressure right now — which locations dry out the fastest, take the most wind, or heat up earliest in the season?
Carry forward: If you’re working through the toolkit linearly, your dry season length and highest-ET area carry into Article 2 as context for mapping your water sources and losses.
RESOURCES
Climate Toolbox — — United States Department of Agriculture (USDA) collection of web tools including Historical Climograph, Climate Mapper, and Future Climate Dashboard.
Climate Data Online — — NOAA archive of global historical weather and climate data including daily, monthly, seasonal, and yearly measurements.
OpenET — — Satellite-based ET data by field.
AgriMet — — Daily temperature, rainfall, and reference ET for Pacific Northwest locations.
Soil Climate Analysis Network — — USDA soil temperature and moisture data from monitoring stations.
DFI × OSU CRAFS · Water Resilience Toolkit · Grower Pathway