how is land surveying related to irrigation ?

Two elements of land surveying can apply to your question

1) Grading and Drainage... here a surveyor would map out the land to determine the best method to facilitate drainage or storm water flow. This is generally just a matter of leveling and topographic mapping, however some aspects such as flooding and Federal Mandated Flood Insurance could also be an aspect of this.

2) Boundary.. if the body of water meets the definition of a navigable water way, then the high water mark defines the boundary line between private and federal property. Thus the surveyor would have to determine that elevation to establish the boundary.

the survey can be for several purposes, one is to determine where the low spots in the field are as they will collect water. another is to set the grade on a supply ditch so water flows the right direction. some fields may be leveled out so the volume of earthwork needs to be calculated and optimum elevations determined. mechanical irrigation systems need to be sized to the field, so you need a survey to get the dimensions and shape. not all fields are "square".