#define ETAPE A0

// Set crucial level as appropriate for your needs.
// For me, 687 indicates three inches of water.
#define CRUCIAL_LEVEL 687
#define LED_RED 13
#define LED_GREEN 12

float resistance;

void setup(void) {
  pinMode(LED_RED, OUTPUT);
  pinMode(LED_GREEN, OUTPUT);
  Serial.begin(9600);
}

void loop(void) {
  resistance = analogRead(ETAPE);
  Serial.println(resistance);

  if (resistance > CRUCIAL_LEVEL) {
    glowRed();
  } else {
    glowGreen();
  }

  delay(1000);
}

void glowRed() {
  digitalWrite(LED_RED, HIGH);
  digitalWrite(LED_GREEN, LOW);
}

void glowGreen() {
  digitalWrite(LED_GREEN, HIGH);
  digitalWrite(LED_RED, LOW);
}