The following points may be helpful:
1. First of all, an introduction won’t hurt:
The graph represents global water consumption from 1900 to 2000. The vertical axis shows the general amount of water consumption in cubic kilometers (mk3). The horizontal axis compares the decades of the 20th century. The graph has been designed to illustrate the global water consumption per 1000 cubic kilometers at ten-year intervals.
2. Now, you can comment on the information it represents, making any comparisons you want:
-A great amount of water supplies was used in agriculture, increasing steadily from the commencement of the 20th century to its end- through the whole period.
-Global water consumption amounted to more that 3,000 mk3 in the 20th century.
3. Your conclusion may be the statement of a fact like the following sentence:
-The larger agricultural land a country has, the greater water reservoirs it will use/need.
-The first curves demonstrate three different usage of global water while the table reveals the data about the consumed water and some relevant issues in Brazil and Congo.
What do you mean by “the first curves”? We can see three curves in the graph, so you have to say “the graph shows….”
By the way, a “curve” illustrates a shape and position.
-Agricultural use reached 3000 km3 in 2000, which was three times as much as the record in 1950. Industrial use in 2000 was more than 1000 km3-twice/two times as much as the domestic consumption today.
-When it comes to the table,
Instead you could say “as for the table”. It sounds better.
-To sum up, agricultural use is the largest proportion of water as per this graph.
I’m not sure what you mean here. You simply could say:
The largest proportion of water was used in agriculture as it is shown in the graph.