Diploma Thesis

Sustainable Agriculture in the Lower Murrumbidgee Catchment

The Murrumbidgee Irrigation Area (MIA) is located in New South Wales, South-eastern Australia. The main occupation in the area is agriculture. Typical products are rice, wheat and fruits as well as meat and wool from sheep farming. MIA suffers from shallow groundwater tables and the resulting salination. The main reason for these problems is the agriculture, above all the rice farming with an intense irrigation. Since the beginning of the 20th century, groundwater tables in the region have risen from 20m under surface up to the surface. From a certain level on, the groundwater evaporates directly leaving salt in the upper parts of the soil.

The objective of this work is to find out how the actual irrigation and cropping management can be changed in order to be sustainable in terms of water and salinity as well as which processes influence the system.

In a first step, a box model with a water balance estimates the behaviour of the groundwater table in MIA. This model demonstrates that a steady state groundwater table exists. It is influenced by the efficiency of the drainage and the evaporation from the groundwater. Another question is, which effect this steady state water table has on the salinity.

In a second step, a refined groundwater model is set up. This model considers a greater area besides MIA, bordered by the Murrumbidgee and the Lachlan River as well as a ridge in the east. The model calculates in a transient flow simulation the elevation of the water table between 1950 and 2030. With these results, the danger of salination is estimated by defining a critical groundwater level of 2 m under surface. The area with shallower water tables is marked as endangered.

With a rice area of 40’000 ha, an area of 42% will be endangered by salination in 2030. With the actual rice area of 32’000 ha (in 2000) the extent of the area endangered by salination will be 27%. The local steady state under rice fields is reached, but regionally, the groundwater is still rising.

Compared with the target, that not more than 20% of the land should become saline, the agriculture is not sustainable. It is recommended to reduce the overall rice growing area.