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HTML 

              
                <div class="canvas-container">
  <canvas id="canvas"></canvas>
</div>
<div class="controls">
  <fieldset>
    <legend>Rain</legend>

    <input type="radio" name="rain" id="no-rain" value="0" />
    <label for="no-rain">☁️</label>

    <input type="radio" name="rain" id="medium-rain" value="1" checked />
    <label for="medium-rain">🌧️</label>
  </fieldset>

  <fieldset>
    <legend>Evaporate</legend>

    <input type="radio" name="evaporation" id="slow-evaporation" value="1" checked />
    <label for="slow-evaporation">🌤️</label>

    <input type="radio" name="evaporation" id="fast-evaporation" value="2" />
    <label for="fast-evaporation">☀️</label>
  </fieldset>
</div>
!

CSS 

              
                body {
  height: 100dvh;
  margin: 0;
  overflow: hidden;
  display: grid;
  place-items: center;
  background: #111;
  color: #eee;
  font: normal 16px/1 "Operator Mono", menlo, monaco, monospace;
}

.canvas-container {
  position: relative;
  width: clamp(200px, 800px, 80vmin);
  height: clamp(200px, 800px, 80vmin);
}

canvas {
  position: absolute;
  width: 100%;
  height: 100%;
  image-rendering: pixelated;
  border-radius: 2em;
  box-shadow: 0 .5em 1em #000a;
  cursor: pointer;
}

.controls {
  display: flex;
  gap: 1em;
}

.controls fieldset {
  border-radius: 0.5em;
  text-align: center;
}

.controls label {
  display: inline-block;
  font-size: 1.5em;
  padding: 0.2em;
  cursor: pointer;
}

.controls input {
  appearance: none;
  opacity: 0;
  position: absolute;
}

.controls input:checked + label {
  border-radius: 50%;
  box-shadow: 0 0 0 2px #ff0a;
}

.controls input:focus + label {
  background: #ff03;
}
!

JS 

              
                const { abs, min, sin, sqrt, pow, PI: π } = Math
const query = document.querySelector.bind(document)
const queryAll = document.querySelectorAll.bind(document)
const canvas = query('canvas')
const width = 130
const height = 130
const scale = 1.6
const surfaceScale = 10
const detailScale = 20
canvas.width = width
canvas.height = height
const ctx = canvas.getContext('2d')
const noop =_=>_
const random = (max = 1) => Math.random() * max
const getArray = (length, mapper) => Array.from(Array(length), mapper || noop)
const calcDistance = (p1, p2) => sqrt(pow(abs(p1.x - p2.x), 2) + pow(abs(p1.y - p2.y), 2))

const tickFPS = 60
const wetness = 0.015
const erosionBase = 0.002

const controls = {
  rain: 1,
  evaporation: 1
}

const generateLand = (width, height) => {
  noise.seed(random())
  const xScale = width / scale
  const yScale = height / scale
  const xSurfaceScale = width / surfaceScale
  const ySurfaceScale = height / surfaceScale
  const xDetailScale = width / detailScale
  const yDetailScale = height / detailScale
  return getArray(height, (_, y) => {
    return getArray(width, (_, x) => {
      const base = noise.simplex2(x / xScale, y / yScale)
      const surface = noise.simplex2(x / xSurfaceScale, y / ySurfaceScale)
      const detail = noise.simplex2(x / xDetailScale, y / yDetailScale)
      return base + surface / 20 + detail / 30
    })
  })
}

const generateCells = (width, height) => {
  return getArray(height, (_, y) => {
    return getArray(width, (_, x) => 0)
  })
}

const resetState = () => {
  return {
    isRaining: false,
    water: [],
    land: generateLand(width, height),
    wetland: generateCells(width, height),
    vegetation: generateCells(width, height),
    frame: 0
  }
}

let state = resetState()

const rain = () => {
  const amount = 80
  state.isRaining = controls.rain === 1 && (state.frame === 0 || random() > .8)
  if (state.isRaining) {
    if (controls.rain === 1 && random() > .5) {
      state.isRaining = false
    } else {
      for (let i = 0; i < amount; i++) {
        const x = parseInt(random(width))
        const y = parseInt(random(height))
        state.water.push({ x, y })
      }
    }
  }
}

const evaporate = () => {
  const waterToKeep = state.water.length / (controls.evaporation === 1 ? 1.02 : 1.5)
  const toSlice = state.water.length - waterToKeep
  state.water = state.water.slice(-waterToKeep)
  if (state.water.length === 1) {
    state.water = []
  }
}

const dry = () => {
  const speed = controls.evaporation === 1 ? 1.0001 : 1.005
  state.wetland.forEach((row, y) => {
    row.forEach((cell, x) => {
      if (!(cell > 0)) {
        return
      }
      const water = state.water.find(water => {
        return x === water.x &&  y === water.y
      })
      if (!water && cell > 0.2) {
        state.wetland[y][x] /= speed
      }
    })
  })
}

const waterflow = () => {
  state.water.forEach(water => {
    if (water.y < 0 || water.y > height - 1) {
      return
    }
    const selfRow = state.land[water.y]
    const aboveRow = state.land[water.y - 1] || selfRow
    const belowRow = state.land[water.y + 1] || selfRow
    const flowMap = [{
      y: -1, x: -1,
      height: aboveRow[water.x - 1] || 1
    }, {
      y: -1, x: 0,
      height: aboveRow[water.x]
    }, {
      y: -1, x: 1,
      height: aboveRow[water.x + 1] || 1
    }, {
      y: 0, x: -1,
      height: selfRow[water.x - 1] || 1
    }, {
      y: 0, x: 0,
      height: selfRow[water.x]
    }, {
      y: 0, x: 1,
      height: selfRow[water.x + 1] || 1
    }, {
      y: 1, x: -1,
      height: belowRow[water.x - 1] || 1
    }, {
      y: 1, x: 0,
      height: belowRow[water.x]
    }, {
      y: 1, x: 1,
      height: belowRow[water.x + 1] || 1
    }]
    const nextFlow = flowMap.sort((cellA, cellB) => cellA.height - cellB.height)[0]
    const blockedByOtherWater = state.water.find(({ x, y }) => {
      const sameX = x === water.x + nextFlow.x
      const sameY = y === water.y + nextFlow.y
      return sameX && sameY
    })
    if (blockedByOtherWater) {
      water.isFlowing = false
      return
    }
    water.isFlowing = true
    state.wetland[water.y][water.x] += (state.wetland[water.y][water.x] < 1 ? wetness : 0)

    const currentHeight = state.land[water.y][water.x]
    const nextHeight = state.land[water.y + nextFlow.y] ? state.land[water.y + nextFlow.y][water.x + nextFlow.x] : currentHeight
    const heightDiff = abs(currentHeight - nextHeight)
    let erosion = erosionBase * heightDiff
    state.land[water.y][water.x] -= (state.land[water.y][water.x] > 0 ? erosion : 0)
    if (state.land[water.y - 1]) {
      state.land[water.y - 1][water.x - 1] -= (state.land[water.y - 1][water.x - 1] > 0 ? erosion/2 : 0)
      state.land[water.y - 1][water.x - 2] -= (state.land[water.y - 1][water.x - 2] > 0 ? erosion/4 : 0)
      state.land[water.y - 1][water.x + 1] -= (state.land[water.y - 1][water.x + 1] > 0 ? erosion/2 : 0)
      state.land[water.y - 1][water.x + 2] -= (state.land[water.y - 1][water.x + 2] > 0 ? erosion/4 : 0)
    }
    if (state.land[water.y + 1]) {
      state.land[water.y + 1][water.x - 1] -= (state.land[water.y + 1][water.x - 1] > 0 ? erosion/2 : 0)
      state.land[water.y + 1][water.x - 2] -= (state.land[water.y + 1][water.x - 2] > 0 ? erosion/4 : 0)
      state.land[water.y + 1][water.x + 1] -= (state.land[water.y + 1][water.x + 1] > 0 ? erosion/2 : 0)
      state.land[water.y + 1][water.x + 2] -= (state.land[water.y + 1][water.x + 2] > 0 ? erosion/4 : 0)
    }
    state.land[water.y][water.x - 1] -= (state.land[water.y][water.x - 1] > 0 ? erosion/1.8 : 0)
    state.land[water.y][water.x - 2] -= (state.land[water.y][water.x - 2] > 0 ? erosion/3.5 : 0)
    state.land[water.y][water.x + 1] -= (state.land[water.y][water.x + 1] > 0 ? erosion/1.8 : 0)
    state.land[water.y][water.x + 2] -= (state.land[water.y][water.x + 2] > 0 ? erosion/3.5 : 0)
    const vSpread0 = [0.0002, 0.00005, 0.00001]
    const vSpread1 = [0.0001, 0.000025, 0.000005]
    const vSpread2 = [0.00005, 0.0000125, 0.0000025]
    state.vegetation[water.y][water.x - 1] += (state.vegetation[water.y][water.x - 1] < 1 ? vSpread0[0] : 0)
    state.vegetation[water.y][water.x - 2] += (state.vegetation[water.y][water.x - 2] < 1 ? vSpread0[1] : 0)
    state.vegetation[water.y][water.x - 3] += (state.vegetation[water.y][water.x - 3] < 1 ? vSpread0[2] : 0)
    state.vegetation[water.y][water.x + 1] += (state.vegetation[water.y][water.x + 1] < 1 ? vSpread0[0] : 0)
    state.vegetation[water.y][water.x + 2] += (state.vegetation[water.y][water.x + 2] < 1 ? vSpread0[1] : 0)
    state.vegetation[water.y][water.x + 3] += (state.vegetation[water.y][water.x + 3] < 1 ? vSpread0[2] : 0)
    if (state.vegetation[water.y - 1]) {
      state.vegetation[water.y - 1][water.x - 1] += (state.vegetation[water.y - 1][water.x - 1] < 1 ? vSpread1[0] : 0)
      state.vegetation[water.y - 1][water.x - 2] += (state.vegetation[water.y - 1][water.x - 2] < 1 ? vSpread1[1] : 0)
      state.vegetation[water.y - 1][water.x - 3] += (state.vegetation[water.y - 1][water.x - 3] < 1 ? vSpread1[2] : 0)
      state.vegetation[water.y - 1][water.x + 1] += (state.vegetation[water.y - 1][water.x + 1] < 1 ? vSpread1[0] : 0)
      state.vegetation[water.y - 1][water.x + 2] += (state.vegetation[water.y - 1][water.x + 2] < 1 ? vSpread1[1] : 0)
      state.vegetation[water.y - 1][water.x + 3] += (state.vegetation[water.y - 1][water.x + 3] < 1 ? vSpread1[2] : 0)
    }
    if (state.vegetation[water.y - 2]) {
      state.vegetation[water.y - 2][water.x - 1] += (state.vegetation[water.y - 2][water.x - 1] < 1 ? vSpread2[0] : 0)
      state.vegetation[water.y - 2][water.x - 2] += (state.vegetation[water.y - 2][water.x - 2] < 1 ? vSpread2[1] : 0)
      state.vegetation[water.y - 2][water.x - 3] += (state.vegetation[water.y - 2][water.x - 3] < 1 ? vSpread2[2] : 0)
      state.vegetation[water.y - 2][water.x + 1] += (state.vegetation[water.y - 2][water.x + 1] < 1 ? vSpread2[0] : 0)
      state.vegetation[water.y - 2][water.x + 2] += (state.vegetation[water.y - 2][water.x + 2] < 1 ? vSpread2[1] : 0)
      state.vegetation[water.y - 2][water.x + 3] += (state.vegetation[water.y - 2][water.x + 3] < 1 ? vSpread2[2] : 0)
    }
    if (state.vegetation[water.y + 1]) {
      state.vegetation[water.y + 1][water.x - 1] += (state.vegetation[water.y + 1][water.x - 1] < 1 ? vSpread1[0] : 0)
      state.vegetation[water.y + 1][water.x - 2] += (state.vegetation[water.y + 1][water.x - 2] < 1 ? vSpread1[1] : 0)
      state.vegetation[water.y + 1][water.x - 3] += (state.vegetation[water.y + 1][water.x - 3] < 1 ? vSpread1[2] : 0)
      state.vegetation[water.y + 1][water.x + 1] += (state.vegetation[water.y + 1][water.x + 1] < 1 ? vSpread1[0] : 0)
      state.vegetation[water.y + 1][water.x + 2] += (state.vegetation[water.y + 1][water.x + 2] < 1 ? vSpread1[1] : 0)
      state.vegetation[water.y + 1][water.x + 3] += (state.vegetation[water.y + 1][water.x + 3] < 1 ? vSpread1[2] : 0)
    }
    if (state.vegetation[water.y + 2]) {
      state.vegetation[water.y + 2][water.x - 1] += (state.vegetation[water.y + 2][water.x - 1] < 1 ? vSpread2[0] : 0)
      state.vegetation[water.y + 2][water.x - 2] += (state.vegetation[water.y + 2][water.x - 2] < 1 ? vSpread2[1] : 0)
      state.vegetation[water.y + 2][water.x - 3] += (state.vegetation[water.y + 2][water.x - 3] < 1 ? vSpread2[2] : 0)
      state.vegetation[water.y + 2][water.x + 1] += (state.vegetation[water.y + 2][water.x + 1] < 1 ? vSpread2[0] : 0)
      state.vegetation[water.y + 2][water.x + 2] += (state.vegetation[water.y + 2][water.x + 2] < 1 ? vSpread2[1] : 0)
      state.vegetation[water.y + 2][water.x + 3] += (state.vegetation[water.y + 2][water.x + 3] < 1 ? vSpread2[2] : 0)
    }
    if (state.vegetation[water.y][water.x - 1] > 0.25 && state.vegetation[water.y][water.x + 1] > 0.25) {
      water.isInLake = true
    }
    water.x += nextFlow.x
    water.y += nextFlow.y
  })
}

const vegetate = () => {
  state.vegetation.forEach((row, y) => {
    row.forEach((cell, x) => {
      let waterNearby = false
      let growth = 0
      for (let i = 0; i < state.water.length - 1; i++) {
        const water = state.water[i]
        const distance = calcDistance(water, { x, y })
        if (distance < 3) {
          waterNearby = water
          growth = ((1 - cell) / 30_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 6) {
          waterNearby = water
          growth = ((1 - cell) / 90_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 9) {
          waterNearby = water
          growth = ((1 - cell) / 150_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 12) {
          waterNearby = water
          growth = ((1 - cell) / 270_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 15) {
          waterNearby = water
          growth = ((1 - cell) / 510_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 18) {
          waterNearby = water
          growth = ((1 - cell) / 990_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 20) {
          waterNearby = water
          growth = ((1 - cell) / 1_950_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 21) {
          waterNearby = water
          growth = ((1 - cell) / 3_830_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 25) {
          waterNearby = water
          growth = ((1 - cell) / 7_590_000) * (1 - state.land[y][x])
          break
        }
        if (distance < 30) {
          waterNearby = water
          growth = ((1 - cell) / 15_110_000) * (1 - state.land[y][x])
          break
        }
      }
      if (!waterNearby) {
        if (cell > 0.3) {
          state.vegetation[y][x] -= 0.01
        } else if (cell >= 0.001) {
          state.vegetation[y][x] -= 0.001
        } else {
          state.vegetation[y][x] = 0
        }
        return
      }
      if (!(cell > 0)) {
        return
      }
      state.vegetation[y][x] += (state.vegetation[y][x] < 1 ? growth : 0)
      state.vegetation[y][x + 1] += (state.vegetation[y][x + 1] < 1 ? growth : 0)
      state.vegetation[y][x - 1] += (state.vegetation[y][x - 1] < 1 ? growth : 0)
      state.vegetation[y][x + 2] += (state.vegetation[y][x + 1] < 1 ? growth / 2 : 0)
      state.vegetation[y][x - 2] += (state.vegetation[y][x - 1] < 1 ? growth / 2 : 0)
      state.vegetation[y][x + 3] += (state.vegetation[y][x + 1] < 1 ? growth / 3 : 0)
      state.vegetation[y][x - 3] += (state.vegetation[y][x - 1] < 1 ? growth / 3 : 0)
      if (state.vegetation[y + 1]) {
        state.vegetation[y + 1][x] += (state.vegetation[y + 1][x] < 1 ? growth : 0)
        state.vegetation[y + 1][x - 1] += (state.vegetation[y + 1][x - 1] < 1 ? growth / 2 : 0)
        state.vegetation[y + 1][x + 1] += (state.vegetation[y + 1][x + 1] < 1 ? growth / 2 : 0)
        state.vegetation[y + 1][x - 2] += (state.vegetation[y + 1][x - 2] < 1 ? growth / 3 : 0)
        state.vegetation[y + 1][x + 2] += (state.vegetation[y + 1][x + 2] < 1 ? growth / 3 : 0)
      }
      if (state.vegetation[y - 1]) {
        state.vegetation[y - 1][x] += (state.vegetation[y - 1][x] < 1 ? growth : 0)
        state.vegetation[y - 1][x - 1] += (state.vegetation[y - 1][x - 1] < 1 ? growth / 2 : 0)
        state.vegetation[y - 1][x + 1] += (state.vegetation[y - 1][x + 1] < 1 ? growth / 2 : 0)
        state.vegetation[y - 1][x - 2] += (state.vegetation[y - 1][x - 2] < 1 ? growth / 3 : 0)
        state.vegetation[y - 1][x + 2] += (state.vegetation[y - 1][x + 2] < 1 ? growth / 3 : 0)
      }
      if (state.vegetation[y + 2]) {
        state.vegetation[y + 2][x] += (state.vegetation[y + 1][x] < 1 ? growth / 2 : 0)
        state.vegetation[y + 2][x - 1] += (state.vegetation[y + 1][x - 1] < 1 ? growth / 3 : 0)
        state.vegetation[y + 2][x + 1] += (state.vegetation[y + 1][x + 1] < 1 ? growth / 3 : 0)
        state.vegetation[y + 2][x - 2] += (state.vegetation[y + 1][x - 2] < 1 ? growth / 4 : 0)
        state.vegetation[y + 2][x + 2] += (state.vegetation[y + 1][x + 2] < 1 ? growth / 4 : 0)
      }
      if (state.vegetation[y - 2]) {
        state.vegetation[y - 2][x] += (state.vegetation[y - 2][x] < 1 ? growth / 2 : 0)
        state.vegetation[y - 2][x - 1] += (state.vegetation[y - 2][x - 1] < 1 ? growth / 3 : 0)
        state.vegetation[y - 2][x + 1] += (state.vegetation[y - 2][x + 1] < 1 ? growth / 3 : 0)
        state.vegetation[y - 2][x - 2] += (state.vegetation[y - 2][x - 2] < 1 ? growth / 4 : 0)
        state.vegetation[y - 2][x + 2] += (state.vegetation[y - 2][x + 2] < 1 ? growth / 4 : 0)
      }
      if (waterNearby.x === x && waterNearby.y === y) {
        if (cell > 0.25 && cell < 1) {
          state.vegetation[y][x] += 0.001
        } else {
          state.vegetation[y][x] /= 1.001
        }
      }
    })
  })
}

const tick = () => {
  evaporate()
  dry()
  rain()
  waterflow()
  vegetate()
  state.frame++
}
setInterval(tick, 1000/tickFPS)

const drawLand = () => {
  state.land.forEach((row, y) => {
    row.forEach((cell, x) => {
      const hue = 40 - cell * 10
      const saturation = 15 + cell * 5
      const lightness = 50 - cell * 20
      ctx.fillStyle = `hsl(${hue}, ${saturation}%, ${lightness}%)`
      ctx.fillRect(x, y, 1, 1)
    })
  })
}

const drawWetland = () => {
  state.wetland.forEach((row, y) => {
    row.forEach((cell, x) => {
      const hue = 50 + cell * 10
      const saturation = 20 + cell * 5
      const lightness = 45 - cell * 5
      const alpha = min(cell, 0.9)
      ctx.fillStyle = `hsla(${hue}, ${saturation}%, ${lightness}%, ${alpha})`
      ctx.fillRect(x, y, 1, 1)
    })
  })
}

const drawWater = () => {
  state.water.forEach(water => {
    let hue = 200
    let saturation = 70.3
    let lightness = 32
    let alpha = 1
    if (water.isFlowing) {
      hue = hue - 2 + random(4)
      saturation = saturation - 2 + random(4)
      lightness = lightness - 7 + random(14)
      alpha = 0.4 + random(0.3)
      if (water.isInLake) {
        hue = 200 + random(5)
        saturation = 20 + random(5)
      }
    }
    const color = `hsla(${hue}, ${saturation}%, ${lightness}%, ${alpha})`
    ctx.fillStyle = color
    ctx.fillRect(water.x, water.y, 1, 1)
  })
}

const drawVegetation = () => {
  state.vegetation.forEach((row, y) => {
    row.forEach((cell, x) => {
      if (!(cell > 0)) {
        return
      }
      let hue = min(200, 70 + cell * 600) - 5 * noise.simplex2(x / detailScale, y / detailScale)
      let saturation = 30 + cell * 5 + 3 * noise.simplex2(x / detailScale, y / detailScale)
      let lightness = 30 - cell * 15 - 3 * noise.simplex2(x / detailScale, y / detailScale)
      let alpha = cell * 20
      const forestBelt = [0.06, 0.175]
      if (cell > forestBelt[0] && cell < forestBelt[1]) {
        hue = (70 + forestBelt[0] * 600) + (sin((forestBelt[1] - cell) * ((π/2)/(forestBelt[1]-forestBelt[0]))) * 10)
        saturation += (sin((forestBelt[1] - cell) * (π/(forestBelt[1]-forestBelt[0]))) * 5)
        lightness -= (sin((forestBelt[1] - cell) * (π/(forestBelt[1]-forestBelt[0]))) * 5)
      }
      // lake
      if (cell > 0.3) {
        hue = hue - 5 + random(10)
        saturation = saturation - 5 + random(10)
        alpha = alpha - 0.1 + random(0.2)
      }
      ctx.fillStyle = `hsla(${hue}, ${saturation}%, ${lightness}%, ${alpha})`
      ctx.fillRect(x, y, 1, 1)
    })
  })
}

const draw = () => {
  drawLand()
  drawWetland()
  drawWater()
  drawVegetation()
  requestAnimationFrame(draw)
}

requestAnimationFrame(draw)

canvas.addEventListener('click', () => {
  state = resetState()
})

Array.from(queryAll('input[name="rain"]')).forEach(input => {
  input.addEventListener('change', () => {
    controls.rain = +input.value
  })
})

Array.from(queryAll('input[name="evaporation"]')).forEach(input => {
  input.addEventListener('change', () => {
    controls.evaporation = +input.value
  })
})
!
999px

Console