The limitation of fresh water and the range of its production pushed the researchers to moderate some facilities in the field of irrigation that is exploiting of water and increasing its use efficiency, that can be regarded as necessary term through the irrigation water to determine the time of suitable irrigation and adding water quantities that are needed by the plant that gives away to save much quantities of water from that water and increasing the irrigated area, that makes the irrigation in the suitable time a great thing to keep the crop production saved. By using the irrigation scheduling, the irrigation management can be chosen in the critical duration to prevent the superfluous addition of water, while there is a quantity of irrigated water can be reused in so other periods of time instead of wasting it. And in this way, the irrigation is launched with a calculated scheduling system in a simple field way and controlled system to save water for the plant needs for irrigation to prevent the plant from thirst during the season. The potato (Solanium tubersum L.) is recognized as one of the vegetable crops that belong to (Solancease) family, the very important globally, because it is full of nutrition and energy as it is full of carbs as well as vitamin C and B1, if comparing it with other crops like wheat, barley, corn or rice, it is recognized as one of the crops that is sensitive to high and low evaporation stress especially in the last growth stage.

When to Irrigate and How Much Water to Add?

     The answer of this question is involved with a practical significance that contributes in increasing the efficiency of irrigation operations management, the water requirements' estimation is recognized as one of the most important steps that should be done while planning to achieve the various irrigation systems, that include irrigation fields requirements, water consumption (CU) or (ET), various water loses and other water quantities that can be added for some purposes such as leaching requirements.

Irrigation Requirements (IR)

   The quantity of water that must be supplied to join the soil moisture with roots' zone to the field capacity. It represents the difference between the soil moisture at the field capacity and the soil moisture while irrigating, and it can be calculated according the following formula:

d=

How Much Water the Plant Needs?

     The efficient irrigation operations start with calculating how much water needed for the plant and then supplying and transferring that quantity. The quantity can be determined by two ways:

1. Calculating the actual water consumption of crop.

2. Calculating the depleted moisture from soil  and trying to retrieve it.

And there are three fundamentals factors that affect the irrigation periods and added quantity of water:

1 The plant's need of water.

2. Availability of irrigation water.

3. The soil capability of water touch at the roots' zone

2.4- Irrigation Scheduling and the Calculations of Water Consumption of Potato

There are four things that affect the irrigation time and the added water quantity to soil:

1. Availability of water.

2. Crop's need of water.

3. The soil's capability to save water at the roots' zone.

The evaporation – Transpiration values are determined by using the evaporation pan type (A) according to the following steps:

1. Measuring daily evaporation from the evaporation pan, as follows:

ETo= KP * Epan …………………..(2)

2. Calculating the actual evaporation – transpiration, according to the following formula:

ETo= kc * ETo……….……………..(3)

3. The values of reduction factors are determined depending on the surface drip irrigation system to modify the water consumption that calculated in formula (3) because the shaded area don’t include the whole experiment land, according

Kr= Ge + 0.5 (1-Ge)………………….(4)