Unity Falling Damage

The following healthbar script is what I have written. Now I want that my player takes falling damage, but I have no clue how I need to do this. Any guidance?

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
public class Healthbar : MonoBehaviour {

	public Image currentHealthbar;
	public Text ratioText;

	private float hitpoint = 150;
	private float maxHitpoint = 150;

	private void Start()
	{
	UpdateHealthbar	();

}
private void UpdateHealthbar()
{
float ratio = hitpoint / maxHitpoint;
currentHealthbar.rectTransform.localScale = new Vector3 (ratio, 1, 1);
ratioText.text = (ratio * 100).ToString(“0”) + ‘%’;
}

	private void TakeDamage(float damage)
	{
	hitpoint -= damage;
	if (hitpoint <0)
	{
		hitpoint = 0;
		Debug.Log ("Dead!");
	}
	UpdateHealthbar ();
}
private void HealDamage(float heal)
{
	hitpoint += heal;
	if (hitpoint >maxHitpoint)
	{
		hitpoint = maxHitpoint;

}
	UpdateHealthbar ();

}
}

You could set 2 Vector3s, setting them after each other to the players position every second, get the distance between them, and voila you have the value of how much your player moves in 1 second. then do a check if your player is grounded and if it’s not give it a variable that tells how long you player has been in the air(float that gets incremented by Time.deltaTime). After that you could check if the player has been in the air long enough to take damage(roughly), then get the last distance between the vectors that tells how much your player was moving in a second before hitting the ground. And then with a basic mathemathical algorithm and a simple RoundToInt() you would be able to get a decent realistic value for the damage. Hope this helps. Feel free to ask for more information.