sorry for misleading naming in "water", it behaves like sand instead

NoitaPhysics.pde

float[][] grid;
float[][] temp;
float scale = 0.12;
int w, h;
boolean add = false;
float touchSize = 10;

void setup() {
  fullScreen();
  colorMode(HSB,1,1,1,1);
  background(0);
  w = floor(width * scale);
  h = floor(height * scale);
  grid = new float[w][h];
  temp = new float[w][h];
  for (int x = 0; x < w; x++) {
    for (int y = 0; y < h; y++) {
      grid[x][y] = 0;
    }
  }
}

void draw() {
  if(random(1) < .2){
    boolean oldAdd = add;
    add = true;
    inputWater(random(w), 1, 1);
    add = oldAdd;
  }
  if(frameCount % 60 == 0){
    for(int x = 0; x < w; x++){
    //  grid[x][h-1] = 0;
    }
  }
  for(int i = 0; i < 1; i++){
    update();
  }
  display();
}

void display(){
  noStroke();
  float rectWidth  = width  / w + 1;
  float rectHeight = height / h + 1;
  for (int x = 1; x < w-1; x++) {
    float screenX = map(x,1,w-1,0,width);
    for (int y = 1; y < h; y++) {
       float screenY = map(y,1,h,0,height);
       float val = grid[x][y];
       fill(color(val));
       rect(screenX, screenY, rectWidth, rectHeight);
    }
  }
}

void update(){
  for (int x = 1; x < w-1; x++) {
    for (int y = h-2; y > 0; y--) {
      float me = grid[x][y];
      float down = grid[x][y+1];
      float botRight = grid[x+1][y+1];
      float botLeft = grid[x-1][y+1];
      
      if(me > down){
        me -= 1;
        down += 1;
      } else if(me > botLeft){
        me -= 1;
        botLeft += 1;
      } else if(me > botRight){
        me -= 1;
        botRight += 1;
      }
      me = constrain(me, 0, 1);
      down = constrain(down, 0, 1);
      botLeft = constrain(botLeft, 0, 1);
      botRight = constrain(botRight, 0, 1);
      
      grid[x][y] = me;
      grid[x][y+1] = down;
      grid[x+1][y+1] = botRight;
      grid[x-1][y+1] = botLeft;
    }
  }
}

void mousePressed(){
  add = !add;
  inputWater(mouseX*scale, mouseY*scale, touchSize);
}

void mouseDragged(){
  inputWater(mouseX*scale, mouseY*scale, touchSize);
}

void inputWater(float x, float y, float size){
  for (int xi = 0; xi < w; xi++) {
    for (int yi = 0; yi < h; yi++) {
      if(dist(x,y,xi,yi) < size){
        grid[xi][yi]= add ? 1:0;
      }
    }
  }
}