EN 
Українська
Polski
Español
Deutsch
日本語
Italiano
Português (BR)
Français
العربية
Nederlands
中文
한국어
हिन्दी
Türkçe
Tiếng Việt
Русский
Bahasa Indonesia
فارسی
বাংলা
ไทย
Filipino
Kiswahili
اردو
Bahasa Melayu
தமிழ்
עברית
Ελληνικά
ਪੰਜਾਬੀ
አማርኛ
မြန်မာဘာသာ
ខ្មែរ
ქართული
नेपाली
Монгол хэл
Հայերեն
සිංහල
ລາວ
Čeština
Română
Magyar
Български
Svenska
Dansk
Suomi
Norsk
Slovenčina
Hrvatski
Српски
Slovenščina
Lietuvių
Latviešu
Eesti
Íslenska
Shqip
Македонски
Bosanski
Malti🌦️🏜️ Drought & Precipitation Cycle — Soil Moisture Bucket Model
Interactive soil-moisture bucket model — stochastic rainfall, evapotranspiration and runoff drive a landscape through wet years and multi-year droughts.
A soil-moisture "bucket" model shows how stochastic rainfall, temperature-driven evapotranspiration and runoff combine over many simulated years to produce wet spells and multi-year droughts. Adjust the sliders and watch the landscape shift between green and brown as soil moisture rises and falls.
🔬 What It Demonstrates
A single-layer water-balance ("bucket") model: rainfall adds water, evapotranspiration removes it at a temperature- and moisture-dependent rate, and runoff caps the reservoir at field capacity. Droughts emerge naturally as runs of years where losses outpace stochastic rainfall.
🎮 How to Use
Set the mean annual rainfall, rainfall variability and temperature sliders, then fast-forward through years with the speed slider. Watch the info bar and the shaded drought bands on the scrolling graph as multi-year droughts appear.
💡 Did You Know?
Real drought indices such as the Palmer Drought Severity Index are built on exactly this kind of soil-moisture bucket accounting — comparing how much water a patch of land "should" have against how much it actually retains.
About the Drought & Precipitation Cycle Simulation
This simulation implements a single-layer "bucket" water-balance model, one of the simplest and most widely used ways to track soil moisture over time. Each simulated day, stochastic rainfall events — arriving at random on some days and skipping others, with amounts drawn from a right-skewed distribution — add water to a soil-moisture reservoir M. Evapotranspiration continuously removes water at a rate that increases with temperature but is throttled down as the soil dries, mirroring how real dry soil can't wick moisture to the surface as fast as wet soil. Whenever the reservoir would exceed its field capacity FC, the excess water is discarded as runoff rather than piling up indefinitely, keeping M bounded between 0 and FC just like a real soil column.
Running this model over many simulated years reveals how droughts actually emerge: not from a single dry day, but from a sustained run of years where evapotranspiration losses outpace stochastic rainfall income, pulling average soil moisture below a drought threshold (here, 30% of field capacity) for multiple consecutive years. The landscape panel recolors from green to brown as this happens, while the scrolling graph shades the drought years so the buildup is visible at a glance — the same logic used by real indices like the Palmer Drought Severity Index.
Frequently Asked Questions
What is a soil-moisture "bucket" model?
It is a simplified water-balance model that treats a patch of soil as a single reservoir, or bucket, with a maximum holding capacity called field capacity. Rain fills the bucket, evapotranspiration drains it, and any water beyond the bucket's capacity spills out as runoff. Despite its simplicity, this approach captures the essential dynamics of soil drying and wetting used in real hydrology and drought monitoring.
How is evapotranspiration calculated here?
Evapotranspiration combines evaporation from the soil surface and transpiration through plant leaves. In this model its maximum rate rises with temperature, since warmer air can hold and pull away more moisture, but the actual rate is scaled down proportionally to how much moisture the soil currently holds. Dry soil simply cannot supply water to the surface as quickly as wet soil, so the loss rate throttles down as the bucket empties.
What is field capacity and why does it matter?
Field capacity is the maximum amount of water a soil can hold against gravity after excess has drained away. It sets the upper bound on the soil-moisture bucket in this simulation. Once soil moisture reaches field capacity, any additional rainfall cannot be stored and instead becomes runoff, which is why very wet periods do not let moisture climb indefinitely.
Why does runoff happen instead of the bucket overflowing forever?
Real soil has a finite capacity to hold water against gravity; once it is saturated, additional rain simply drains through or flows overland as runoff rather than being stored. The simulation enforces this by capping soil moisture at field capacity each simulated day and discarding any excess, which keeps the reservoir physically realistic instead of growing without bound.
How does the simulation decide a year is a "drought year"?
Each simulated year's average soil moisture is compared against a drought threshold set at 30% of field capacity. If the yearly average falls below that threshold, the year is flagged as a drought year and shaded on the scrolling time-series graph. A true multi-year drought becomes visible as a run of consecutive shaded years, not just a single dip.
What do the rainfall sliders actually control?
The mean annual rainfall slider sets the long-run average precipitation that the random rain generator targets over many years. The rainfall variability slider controls how bursty and unevenly distributed individual rain events are — low variability spreads rainfall evenly across the year, while high variability produces long dry stretches interrupted by intense storms, which is how many real semi-arid climates behave.
Is this how real drought forecasting works?
Yes, in essence. Widely used indices such as the Palmer Drought Severity Index and land-surface models used in weather and climate forecasting are built on similar soil-moisture bucket accounting, tracking the balance between incoming precipitation and outgoing evapotranspiration over time. This simulation is a simplified, single-layer version of that same core idea.
Why does raising the temperature slider make droughts worse?
Higher temperatures increase the maximum possible evapotranspiration rate, since warmer air can absorb and carry away more water vapor from soil and plants. With rainfall unchanged, a higher evapotranspiration rate drains the soil-moisture bucket faster, pushing the average moisture level down and making below-threshold drought years more likely and more severe.
Interactive soil-moisture bucket model — stochastic rainfall, evapotranspiration and runoff drive a landscape through wet years and multi-year droughts.
2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install